Support `requires_python` field to detect Python 3 support

[fmigneault]

Multiple packages providers prefer to define requires_python: ">=3.7" instead of individual Programming Language :: Python classifiers for each new version supported.

For example:
pallets/werkzeug#2528

pyup should detect this field when provided on pypy (eg: https://pypi.org/pypi/werkzeug/json) to avoid flagging packages as not supporting Python 3, when they are in fact strictly requesting it.

This would resolve similar problems identified in #202, #269