Some wheels only include METADATA files (examples: wheel 0.31.1,
tenacity 4.10.0). Previously whl did not parse the requirement
sections. Add code to parse these sections.

The most complicated part is parsing out the extra == 'something'
clauses from the boolean expression. Use the same expression parser
used by pkg_resources.evaluate_marker which is already used by this
code to find those clauses and produce a version with them removed.

This commit also added --python_interpreter argument to piptool.py to
support this attribute.

Example usage:
pip_import(
    name = 'pip_deps',
    requirements = '//:requirements.txt',
    python_interpreter = 'python3.7',
)

whl: Parse requirements from METADATA files

Add optional attr python_interpreter to run pip with python 3