When downloading a Python package from, say, a zipfile using http,
pip will report the proper version number, but the requirements
file will have a URL, which will cause a mismatch.
As an example, consider requirements.txt that contains:
pip freeze will report:
Running piplint will cause an error:
$ piplint requirements.txt
Requirement 'https://github.com/dcramer/piplint/zipball/master' not found in virtualenv.
You must correct your environment before committing (and running tests).