extras_require is ignored when used with pbr

In normal setup.py you can specify extras_require. However, it's ignored when used together with pbr. Example:

setup(
    setup_requires=["pbr"],
    pbr=True,
    extras_require={"full": ["requests"]}
)

When you run:

pip install .[full]

you get this warning:
WARNING: xxx 0.2.0 does not provide the extra 'full'

I created simple fix here: https://github.com/jozo/pbr/commit/7853fdbbc6d5b8bb3a97398dbe24ccb2b39b90e8