Pip-requirements-parser

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Ensure all tests pass correctly.
Adopt latest skelton

Add new RequirementsFile.from_string() convenience factory method

Vendor LegacyVersion from pre V2 packaging. Otherwise packaging v2 broke
this library and its dependencies.

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Emergency pin of packaging to version under 22.
This breaks this library and its dependents otherwise

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Add new tests. No other changes.

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Add new convenience method InstallRequirement.get_pinned_version() to return
the pinned version if there is such thing.

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Fix twine warning wrt. long_description content type.

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Include code in wheel. This was not included otherwise.
Improve documentation