Partially handle extra version for versions constraint

[mkniewallner]

Resolves: python-poetry/poetry#3186

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

Poetry seems to assume that all packages follow SemVer specification for versioning. However, some packages do not, as python-poetry/poetry#3186 highlights with anchor-exp.

Per PEP 440 specification, when packages define more than 3 parts for the versioning, using ~= 1.4.5.0 is equivalent to >= 1.4.5.0, == 1.4.5.*.

Poetry doesn't handle that correctly today, since defining ~= 1.4.5.0 means that we accept >= 1.4.5.0, == 1.4.*.*.

This PR is an attempt at solving this case for both ~= and wildcard operator. It is in draft for multiple reasons:

• It only handles one more level, which is useful for dependencies using a 4-parts versioning, but it still doesn't handle versioning using more than 4 parts (which I don't think is common, but this could still happen in theory).
• it only handles ~= and wildcard operators, but what about other ones? Per the documentation, ^ is only for SemVer, but this is less clear for ~, though given how it works, it also doesn't seem to be a good fit with something else than SemVer.
• Finally, since this would be a breaking change in the constraints handling, I'm not sure we want to introduce this change.

Side note: python-poetry/poetry#3186 also highlights the fact that ~= is undocumented in Poetry. Is there a particular reason to not document it, or would a PR documenting it would be accepted?

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[sonarcloud]

Kudos, SonarCloud Quality Gate passed!   

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