Install either mkdocs or sphinx depending on document type

[bsipocz]

This should address #2975 (comment)

@agjohnson - I'm not familiar with the mkdocs builds. Is sphinx and sphinx_rtd_theme required to be installed when the document type is mkdocs? If not, I can add a similar check for those dependencies.

[ericholscher]

This looks like a great change. I merged @willingc's simple fix, but I like this one going forward. We should definitely be selecting the requirements based on project type, for both Conda & Pip. So +1 on doing the larger bit of work, and will deploy the simple fix now.

[bsipocz]

Great. Could you confirm that for mkdocs, neither of the above listed sphinx packages are required?

[willingc]

I agree that @bsipocz's suggested solution is the best way to go forward. You should not need sphinx or the sphinx rtd theme with mkdocs.

[willingc]

https://github.com/mkdocs/mkdocs/blob/master/requirements/project.txt FYI @bsipocz

[ericholscher]

Yep, mkdocs & Sphinx don't depend on each other at all, so both can be uninstalled in each environment.

[bsipocz]

Thanks. I should have rtfd at least here of all places :)

[ericholscher]

Thanks!