Add python-coveralls entrypoint

[ericdill]

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Hey @TheKevJames -- what do you think about adding another entrypoint python-coveralls? We're using cpp-coveralls in one of our projects and that library has both the coveralls entrypoint as well as the cpp-coveralls entrypoint. If both coveralls-python and cpp-coveralls are installed together then the coveralls CLI is non-deterministic. One way around that would be to add an explicit python-coveralls (or coveralls-python) entrypoint to the setup.py here so that users can specifically select which coveralls CLI they want to use. Thoughts? If you approve I'm happy to submit PRs here and to the conda-forge feedstock.

https://github.com/eddyxu/cpp-coveralls/blob/master/setup.py#L47-L48

[TheKevJames]

Hi @ericdill, I would be comfortable accepting a PR which adds the python-coveralls entrypoint.