parse_setup_py: Handle string dependency when we expect list of strings

[jherland]

As described in issue #429, the s3transfer package comes with setup.py
where the extras_require dictionary does not map to a list of strings
(extra requirements), but instead map to a string directly.

When s3transfer is installed with this extra (e.g. pip install s3transfer[crt]), the setuptools machinery handles this variation
gracefully, i.e. it interprets the string as if it was a single-element
list containing the same string.

An experiment reveals that the same interpretation occurs if a string is
passed directly in the install_requires argument.

This commit teaches FawltyDeps to do the same.

Also, if parse_one_req() fails to parse a requirement string, catch that
exception (likely an InvalidRequirement or some other subclass of
ValueError), and convert it into a DependencyParsingError, which is
handled more gracefully (i.e. print a warning message and continue).

[mknorps]

Thank you @jherland !

We are getting deeper into the rabbit hole, but at least we are closer to how things works under the Python hood.

I have no comments about the code, all nicely documented and improving the codebase 🚀