Uppercase "extra" dependency marker not working

[jeroendecroos]

Description

When installing a dependency with an "extras" specified ("dependency[extra]" https://www.python.org/dev/peps/pep-0508/#id12) it fails to install the requirement.

Pip does not fail, but a line in the logs show the issue:
Ignoring pystache: markers 'extra == "Mustache"' don't match your environment

I found some related issues:
#4617
#3810

But this seems either a regression or an unhandled case.

I've also started the debugger to confirm more:

(Pdb) l
454
455         def _make_requirement_from_install_req(
456             self, ireq: InstallRequirement, requested_extras: Iterable[str]
457         ) -> Optional[Requirement]:
458             import pdb; pdb.set_trace()
459  ->         if not ireq.match_markers(requested_extras):
460                 logger.info(
461                     "Ignoring %s: markers '%s' don't match your environment",
462                     ireq.name,
463                     ireq.markers,
464                 )
(Pdb) p requested_extras
frozenset({'mustache'})
(Pdb) p ireq
<InstallRequirement object: pystache; extra == "Mustache" (from gitchangelog[Mustache]) editable=False>
(Pdb) p  ireq.match_markers(requested_extras)
False
(Pdb) p  ireq.match_markers(frozenset({"Mustache"}))
True
(Pdb)

I'll probably can open up a PR to fix this soon.

Expected behavior

It installs the extra dependencies requested

pip version

22.0.3

Python version

3.7

OS

CentOS7

How to Reproduce

$ pip --version
pip 22.0.3 from [SCRUBED]/test_venv/lib/python3.7/site-packages/pip (python 3.7)
$ pip install gitchangelog[Mustache]

Output

Collecting gitchangelog[mustache]
  Downloading https://files.pythonhosted.org/packages/9d/f9/554f9d1e2031a330148299b81ed7d6d5f1ef8969154384ca4ae8dd403b7a/gitchangelog-3.0.4-py2.py3-none-any.
whl (54kB)
     |████████████████████████████████| 61kB 7.9MB/s
  Ignoring pystache: markers 'extra == "Mustache"' don't match your environment
Installing collected packages: gitchangelog
Successfully installed gitchangelog-3.0.4

Code of Conduct

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[jeroendecroos]

Seems the solution should be done in packaging I'll open an issue and link

[pradyunsg]

Closing this, since this needs to be handled in packaging and is related to existing Python Packaging standards.