Add optional requires_python to third party stubs metadata

[hamdanal]

The typeshed part of #10722

This PR implements the following:

• parse an optional requires_python field in the third party stubs
• the field must be a valid Python version specifier that specifies the minimum Python version supported by the stub
• the version must be at least the minimum version supported by typeshed (currently 3.7)
• if the field exists, mypy, regression, and stubtest tests are skipped on Python versions that do not satisfy the requirement

stub-uploader still needs to learn about this field and include it in the setup.py file of the corresponding package with a fallback to the minimal version supported by typeshed.