Fix PackageFinder to respect allow_all_prereleases

[micheleAlberto]

Fix 'PackageFinder.find_all_candidates' to respect 'allow_all_prereleases'

Closes #5175

This is another PR following #5927 to address comments raised by @benoit-pierre to ensure that if there is a pre-release in the requirements then prerelease packages are allowed to be installed.

[pganssle]

This supercedes #5927, right?

[pganssle]

@micheleAlberto I believe this is currently failing due to linting. Can you run flake8 over your PR and fix anything that's raising warnings?

[benoit-pierre]

Not just linting. Use tox -e 'lint-{py2,py3}' to run flake. And I suggest running the testsuite at least with your current interpreter and address the failures before pushing:

• tox -e py -- -m unit to run the unit tests
• tox -e py -- -m integration to run the integration tests (this take some time, pytest-xdist is avaliable, so you append something -n3 to distribute the test load on 3 processes)

[uranusjr]

I wonder if this is the correct fix. find_all_candidates is supposed to do what the name indicates—find all possible candidates. Version filtering is performed later down the line (immediately after, in fact, by req.specifier.filter).

pip can already exclude prereleases correctly when you pip install/download/etc., the only one that behaves incorrectly is the check for latest pip. This should be fixed by modifying how pip_version_check uses the result of find_all_candidates instead, IMO.

[cjerdonek]

The comments by @uranusjr seem valid. @micheleAlberto, do you have any reply / do you agree?

[BrownTruck]

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