Merge any marker constraints into constraints with specific markers

[radoering]

Pull Request Check List

Resolves: #4516

• Added tests for changed code.
• Updated documentation for changed code.

If there are several requirements for the same library, requirements with markers can be considered as additions to requirements without markers.

For example see this extract from the install_requires of black 21.9b0:

"typing_extensions>=3.10.0.0",
# 3.10.0.1 is broken on at least Python 3.10,
# https://github.com/python/typing/issues/865
"typing_extensions!=3.10.0.1; python_version >= '3.10'",

These statements define that typing_extensions has to be at least 3.10.0.0 for all python versions (including python 3.10 and above). Additionally, typing_extensions for python 3.10 and above must not be 3.10.0.1.

Thus, the constraints of requirements without markers have to be merged into the constraints of requirements with markers.

[github-actions]

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