Fix pip_cert pytest fixture

[frenzymadness]

pip_cert is recognized as a generator so it should not use
return if PIP_CERT is in os.environ.

Fixes: #2048

• ran the linter to address style issues (tox -e fix_lint)
• wrote descriptive pull request text
• ensured there are test(s) validating the fix
• added news fragment in docs/changelog folder
• updated/extended the documentation

I'm not sure this is testable easily given the fact that a CA cert bundle might be in various locations. Is a change in tests worth mentioning in the changelog?

Before this change:

$ PIP_CERT=/etc/ssl/certs/ca-bundle.crt tox -e py39 -- -x
…
ValueError: pip_cert did not yield a value

after this change:

$ PIP_CERT=/etc/ssl/certs/ca-bundle.crt tox -e py39 -- -x
…
  py39: commands succeeded
  congratulations :)

[codecov]

Codecov Report

Merging #2083 (c082ee5) into main (bb446cb) will decrease coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #2083      +/-   ##
==========================================
- Coverage   93.72%   93.70%   -0.03%     
==========================================
  Files          88       88              
  Lines        4383     4383              
==========================================
- Hits         4108     4107       -1     
- Misses        275      276       +1     

Flag	Coverage Δ
tests	93.70% <ø> (-0.03%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/virtualenv/util/path/_pathlib/__init__.py	93.93% <0.00%> (-3.04%)	⬇️

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[gaborbernat]

Can you add some tests to this, please?

[frenzymadness]

Can you add some tests to this, please?

Do you know about a universal CA bundle I can use across all the systems you have in CI? It needs to be set before pytest is executed so I can imagine a new tox environment running only some tests with this environment variable set.

Is it worth it given the fact that the whole fixture is just a workaround for a problem in pip and its author did not add tests for it?

I can also imagine some dummy test exercising just the unused part of the fixture and actually testing nothing.

[gaborbernat]