markers: add new method to reduce a marker by a Python constraint

[radoering]

That is useful for removing redundant information from markers. For example, if the project's Python constraint is >=3.9, a dependency's marker like python_version >= 3.7 carries the same information as an AnyMarker for this project. (The dependency is required for all supported Python versions.)

Required for python-poetry/poetry#9427

• [x] Added tests for changed code.
• [ ] Updated documentation for changed code.