Fix tests on Windows

[NathanUrwin]

$ pipenv run inv tests
usage: pytest [options] [file_or_dir] [file_or_dir] [...]
pytest: error: unrecognized arguments: --cov=.
  inifile: None
  rootdir: C:\Users\user\Projects\nathanurwin\cookiecutter-git

[NathanUrwin]

pytest-dev/pytest#3434

A current work-around is to $ choco install python3 and $ pip install --user -r requirements.txt. Then you can run $ invoke tests.

[NathanUrwin]

The complete fix after rereading that pytest issue is to $ pipenv run pip install -r requirements.txt. Then you can $ pipenv run invoke tests. I'll make sure to add it to the Contributors section of the README! 👍