Handle "extras" from PEP 508

[gordonmessmer]

https://www.python.org/dev/peps/pep-0508/#extras

[gordonmessmer]

Given $ pip install 'cairocffi>=0.9[xcb]', pip will install cairocffi and seems to treat "[xcb]" as an extras specification, but pkg_resources doesn't seem to parse that requirement...

>>> from pkg_resources import Requirement
>>> parsed_req = Requirement.parse('cairocffi>=0.9[xcb]')
>>> parsed_req.key
'cairocffi'
>>> parsed_req.specs
[('>=', '0.9[xcb]')]
>>> parsed_req.extras
()

>>> parsed_req = Requirement.parse('cairocffi [xcb] >=0.9')
>>> parsed_req.key
'cairocffi'
>>> parsed_req.specs
[('>=', '0.9')]
>>> parsed_req.extras
('xcb',)

[encukou]

According to PEP 508, an extra needs to be right after the name, not after the version.

pip is not a reference implementation of the standards. It has lots of legacy behavior that the standards try to clean up.
Same goes for setuptools/pkg_resources (which seems right in this particular case).
It's best to avoid both in favor of packaging:

>>> import packaging.requirements
>>> packaging.requirements.Requirement('cairocffi>=0.9[xcb]').extras
set()
>>> packaging.requirements.Requirement('cairocffi[xcb]>=0.9').extras
{'xcb'}

Instead of adding pip compatibility, let's fix the packages/projects that use nonstandard specifications.

[hroncok]

We have came to the same conclusion in https://bugzilla.redhat.com/show_bug.cgi?id=1758141#c12 and below.