test_generates_c_extensions fails w/ pypy3

When running the tests with pypy3, test_generates_c_extensions fails, probably due to assuming wrong .so filename for PyPy3 (it's 'testext.pypy36-pp73-x86_64-linux-gnu.so'):

(tox -e pypy3)

```
pbr.tests.test_packaging.TestPackagingWheels.test_generates_c_extensions
------------------------------------------------------------------------

Captured traceback:
~~~~~~~~~~~~~~~~~~~
    Traceback (most recent call last):

      File "/tmp/pbr/pbr/tests/test_packaging.py", line 405, in test_generates_c_extensions
    self.assertTrue(os.path.exists(static_object_path))

      File "/tmp/pbr/.tox/pypy3/site-packages/unittest2/case.py", line 702, in assertTrue
    raise self.failureException(msg)

    AssertionError: False is not true

Captured stdout:
~~~~~~~~~~~~~~~~
    Initialized empty Git repository in /tmp/tmp1ho9j71a/tmpy08zj61n/testpackage/.git/

running bdist_wheel
running build
running build_py
Running custom build_py command.
creating build
creating build/lib.linux-x86_64-3.6
creating build/lib.linux-x86_64-3.6/pbr_testpackage
copying pbr_testpackage/__init__.py -> build/lib.linux-x86_64-3.6/pbr_testpackage
copying pbr_testpackage/_setup_hooks.py -> build/lib.linux-x86_64-3.6/pbr_testpackage
copying pbr_testpackage/cmd.py -> build/lib.linux-x86_64-3.6/pbr_testpackage
copying pbr_testpackage/extra.py -> build/lib.linux-x86_64-3.6/pbr_testpackage
copying pbr_testpackage/wsgi.py -> build/lib.linux-x86_64-3.6/pbr_testpackage
running build_ext
building 'pbr_testpackage.testext' extension
creating build/temp.linux-x86_64-3.6
creating build/temp.linux-x86_64-3.6/src
gcc -pthread -fPIC -I/usr/lib/pypy3.6/include -c src/testext.c -o build/temp.linux-x86_64-3.6/src/testext.o
gcc -pthread -shared build/temp.linux-x86_64-3.6/src/testext.o -o build/lib.linux-x86_64-3.6/pbr_testpackage/testext.pypy36-pp73-x86_64-linux-gnu.so
installing to build/bdist.linux-x86_64/wheel
running install
[pbr] Writing ChangeLog
[pbr] Generating ChangeLog
[pbr] ChangeLog complete (0.0s)
[pbr] Generating AUTHORS
[pbr] AUTHORS complete (0.0s)
running install_lib
creating build/bdist.linux-x86_64
creating build/bdist.linux-x86_64/wheel
creating build/bdist.linux-x86_64/wheel/pbr_testpackage
copying build/lib.linux-x86_64-3.6/pbr_testpackage/testext.pypy36-pp73-x86_64-linux-gnu.so -> build/bdist.linux-x86_64/wheel/pbr_testpackage
copying build/lib.linux-x86_64-3.6/pbr_testpackage/wsgi.py -> build/bdist.linux-x86_64/wheel/pbr_testpackage
copying build/lib.linux-x86_64-3.6/pbr_testpackage/extra.py -> build/bdist.linux-x86_64/wheel/pbr_testpackage
copying build/lib.linux-x86_64-3.6/pbr_testpackage/cmd.py -> build/bdist.linux-x86_64/wheel/pbr_testpackage
copying build/lib.linux-x86_64-3.6/pbr_testpackage/_setup_hooks.py -> build/bdist.linux-x86_64/wheel/pbr_testpackage
copying build/lib.linux-x86_64-3.6/pbr_testpackage/__init__.py -> build/bdist.linux-x86_64/wheel/pbr_testpackage
running install_data
creating build/bdist.linux-x86_64/wheel/pbr_testpackage-0.0.data
creating build/bdist.linux-x86_64/wheel/pbr_testpackage-0.0.data/data
creating build/bdist.linux-x86_64/wheel/pbr_testpackage-0.0.data/data/testpackage
creating build/bdist.linux-x86_64/wheel/pbr_testpackage-0.0.data/data/testpackage/data_files
copying data_files/a.txt -> build/bdist.linux-x86_64/wheel/pbr_testpackage-0.0.data/data/testpackage/data_files/
copying data_files/b.txt -> build/bdist.linux-x86_64/wheel/pbr_testpackage-0.0.data/data/testpackage/data_files/
copying data_files/c.rst -> build/bdist.linux-x86_64/wheel/pbr_testpackage-0.0.data/data/testpackage/data_files/
running install_egg_info
running egg_info
creating pbr_testpackage.egg-info
writing pbr_testpackage.egg-info/PKG-INFO
writing dependency_links to pbr_testpackage.egg-info/dependency_links.txt
writing entry points to pbr_testpackage.egg-info/entry_points.txt
writing requirements to pbr_testpackage.egg-info/requires.txt
writing top-level names to pbr_testpackage.egg-info/top_level.txt
writing pbr to pbr_testpackage.egg-info/pbr.json
[pbr] Processing SOURCES.txt
writing manifest file 'pbr_testpackage.egg-info/SOURCES.txt'
[pbr] In git context, generating filelist from git
reading manifest template 'MANIFEST.in'
writing manifest file 'pbr_testpackage.egg-info/SOURCES.txt'
Copying pbr_testpackage.egg-info to build/bdist.linux-x86_64/wheel/pbr_testpackage-0.0-py3.6.egg-info
running install_scripts
Installing pbr_test_wsgi script to build/bdist.linux-x86_64/wheel/pbr_testpackage-0.0.data/scripts
Installing pbr_test_wsgi_with_class script to build/bdist.linux-x86_64/wheel/pbr_testpackage-0.0.data/scripts
adding license file "LICENSE.txt" (matched pattern "LICEN[CS]E*")
adding license file "AUTHORS" (matched pattern "AUTHORS*")
creating build/bdist.linux-x86_64/wheel/pbr_testpackage-0.0.dist-info/WHEEL
creating 'dist/pbr_testpackage-0.0-pp373-pypy36_pp73-linux_x86_64.whl' and adding 'buil
    d/bdist.linux-x86_64/wheel' to it
adding 'pbr_testpackage/__init__.py'
adding 'pbr_testpackage/_setup_hooks.py'
adding 'pbr_testpackage/cmd.py'
adding 'pbr_testpackage/extra.py'
adding 'pbr_testpackage/testext.pypy36-pp73-x86_64-linux-gnu.so'
adding 'pbr_testpackage/wsgi.py'
adding 'pbr_testpackage-0.0.data/data/testpackage/data_files/a.txt'
adding 'pbr_testpackage-0.0.data/data/testpackage/data_files/b.txt'
adding 'pbr_testpackage-0.0.data/data/testpackage/data_files/c.rst'
adding 'pbr_testpackage-0.0.data/scripts/pbr_test_wsgi'
adding 'pbr_testpackage-0.0.data/scripts/pbr_test_wsgi_with_class'
adding 'pbr_testpackage-0.0.dist-info/AUTHORS'
adding 'pbr_testpackage-0.0.dist-info/LICENSE.txt'
adding 'pbr_testpackage-0.0.dist-info/METADATA'
adding 'pbr_testpackage-0.0.dist-info/WHEEL'
adding 'pbr_testpackage-0.0.dist-info/entry_points.txt'
adding 'pbr_testpackage-0.0.dist-info/pbr.json'
adding 'pbr_testpackage-0.0.dist-info/top_level.txt'
adding 'pbr_testpackage-0.0.dist-info/RECORD'
removing build/bdist.linux-x86_64/wheel
warning: no previously-included files found matching '.gitignore'
warning: no previously-included files found matching '.gitreview'
warning: no previously-included files matching '*.pyc' found anywhere in distribution
/tmp/pbr/pbr/packaging.py:459: EasyInstallDeprecationWarning: Use get_header
  header = easy_install.get_script_header("", executable, is_wininst)
```