Switch to clang for osx, add pytest for test running

[bp-kelley]

This fixes builds on osx. We need to use "clang" not "c" and purest is required for test running now.

[conda-forge-webservices[bot]]

Hi! This is the friendly automated conda-forge-linting service.

I wanted to let you know that I linted all conda-recipes in your PR (recipe) and found some lint.

Here's what I've got...

For recipe:

• Selectors are suggested to take a <two spaces>#<one space>[<expression>] form. See lines [28, 29, 87, 88]

For recipe:

• This recipe is using a compiler, which now requires adding a build dependence on {{ stdlib("c") }} as well. Note that this rule applies to each output of the recipe using a compiler. For further details, please see https://github.com/conda-forge/conda-forge.github.io/issues/2102.

[bp-kelley]

Looks like something else was messed up on my system, I have it resolved now. We still may need pytest though

[greglandrum]

Looks like something else was messed up on my system, I have it resolved now. We still may need pytest though

I don't think that's likely: we don't run the python tests as part of the conda-forge builds. RDKit correctness is tested in the RDKit repository and we don't need to repeat that again here