Fixes pipenv lock nondeterminism with environment markers

[dqkqd]

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The issue

Fixes #5239

The fix

Packages are added into resolver in this order:

1. Load from Pipfile
2. Pass into environment variables PIPENV_PACKAGES
3. Pass into Resolver with the name constraints.
4. Parsed by pip's parsed_requirements
5. Convert into InstallRequirement by install_req_from_parsed_requirement
6. Pass into pip's resolver

After that, we would have all package's markers from resolver.

Thanks @bakhtiary for providing a nice way to overcome this issue by sorting packages.
I think if we want to sort packages, we better sort them after step 5, which is the last step we could intervene.

But even if those packages are sorted, this might not be the best solution. Because markers are generated by pip's resolver, which is something we couldn't control (for example those packages might be reordered inside resolver).

The checklist

• Associated issue
• A news fragment in the news/ directory to describe this fix with the extension .bugfix.rst, .feature.rst, .behavior.rst, .doc.rst. .vendor.rst. or .trivial.rst (this will appear in the release changelog). Use semantic line breaks and name the file after the issue number or the PR #.

[dqkqd]

I don't know how to write a test for this one. Testing by running a loop seems heuristic.
I ran on my laptop without 4b9fc02, and look like it works fine, so just sort self._constraints might be enough for this issue.

./stress-lock.sh
attempt 1: greenlet markers="platform_python_implementation == 'CPython'"
attempt 2: greenlet markers="platform_python_implementation == 'CPython'"
attempt 3: greenlet markers="platform_python_implementation == 'CPython'"
attempt 4: greenlet markers="platform_python_implementation == 'CPython'"
attempt 5: greenlet markers="platform_python_implementation == 'CPython'"
attempt 6: greenlet markers="platform_python_implementation == 'CPython'"
attempt 7: greenlet markers="platform_python_implementation == 'CPython'"
attempt 8: greenlet markers="platform_python_implementation == 'CPython'"
^C

[matteius]

Please rename this file to 5299.bugfix.rst to avoid the merge conflict.

[dqkqd]

Sorry, I didn't check that. Fixed.