we did two four test rebuilds for the whole archive:

http://qa.ubuntuwire.org/ftbfs/rebuilds/test-rebuild-20161216-trusty.html
http://qa.ubuntuwire.org/ftbfs/rebuilds/test-rebuild-20161216-updates-trusty.html

http://qa.ubuntuwire.org/ftbfs/rebuilds/test-rebuild-20161216-python2712-trusty.html
http://qa.ubuntuwire.org/ftbfs/rebuilds/test-rebuild-20161216-updates-python2712-trusty.html

the latter two with the python2.7, python-defaults. python-stdlib-extensions, python-eventlet and python-gevent packages from the ubuntu-toolchain-r/ppa archive.

Comparing the build failures, I found the following regressions:

archivemail	0.9.0-1build1			i386 (maybe TODO)
  python related, disabled test in next upload
  https://launchpad.net/ubuntu/+source/archivemail/0.9.0-1.1
bzr		2.6.0+bzr6593-1ubuntu1		any (ok in updates)
cinder		1:2014.1-0ubuntu1		i386 (ok with updated kombu)
commons-exec	1.2-1				all (test failures, java)
docsis		0.9.6+git16-g61ee500+dfsg-2build1 ppc64el (unrelated, GCC ICE)
eclipse		3.8.1-5.1			arm64 (unrelated, OpenJDK issue?)
extsmail	1.4-1				arm64 (unrelated, GCC ICE)
gcc-defaults	1.124ubuntu6			i386 (unrelated)
  gcj out of memory error
glance		1:2014.1-0ubuntu1		all (ok with updated kombu)
indicator-sound	12.10.2+14.04.20140401-0ubuntu1	i386 (looks unrelated)
  test failures
initramfs-tools-ubuntu-touch	0.72		i386 amd64 armhf (TODO)
 TODO: E: Release signed by unknown key (key id 1E9377A2BA9EF27F)
kombu		3.0.7-1ubuntu1			i386 (ok, updated)
libcrypto++	5.6.1-6				armhf (ok in updates)
liblayout	0.2.10-2			i386 (unrelated)
  OpenJDK out of memory, works when built on amd64)
mgltools-pmv	1.5.7~rc1~cvs.20130519-2	all (TODO)
  pyversions: error parsing Python-Version attribute
nova		1:2014.1-0ubuntu1		i386 (ok with updated kombu)
nuitka		0.5.0.1+ds-1			all (TODO)
  test errors with new python2.7
obexftp		0.23-1.2ubuntu3			any (TODO)
  build error with -Werror=format-security
oslo.messaging	1.3.0-0ubuntu1			all (ok with updated kombu)
pyfltk		1.3.0-1				any (TODO)
  build error with -Werror=format-security
pymol		1.7.0.0-1			any (TODO)
  build error with -Werror=format-security
python-django	1.6.1-2				all (TODO)
  SyntaxError: Non-ASCII character '\xc6' in file test_jslex.py on line 26,
  but no encoding declared
python-eventlet	0.13.0-1ubuntu2			i386 (ok in proposed update)
  0.13.0-1ubuntu2.3
python-glanceclient	1:0.12.0-0ubuntu1	all (TODO)
python-pywcs	1.11-1				any (TODO)
  build error with -Werror=format-security
rawdog		2.19-1				all (TODO)
  Error while fetching feed:
  <urlopen error ('_ssl.c:574: The handshake operation timed out',)>
ruby-lapack	1.5-2				ppc64el (unrelated, ruby)
ruby-timers	1.1.0-1				i386 (unrelated, ruby)
sauerbraten	0.0.20130203.dfsg-1		arm64 (unrelated GCC ICE)
shinken		1.4-2				any
  pyversions: error parsing Python-Version attribute
tora		2.1.3-2build2			arm64 (unrelated, GCC ICE)
wxwidgets2.8	2.8.12.1+dfsg-2ubuntu2		any (unrelated, GCC ICE)
  dump available


With the kombu update in trusty-proposed, there is only the python-glanceclient packages which shows regressions in main.  There are a dozen or so packages in universe which show regressions as well, but on a first glance these look all fixable.

I didn't investigate if the compiler ICEs are really related to the python2.7 update.  These seem to persist even when giving back the packages on the buildds.