Add basic environment marker support to conda skeleton pypi

[evhub]

Looks for simple environment markers of the form ^:python_version(<|<=|!=|==|>=|>)(.+)$ and adds in their requirements if the current Python version matches the specification. Resolves #1974.

[msarahan]

Looks good. Could you please add some tests so that we're actually exercising your addition (mostly to make sure that no one, especially me, breaks your new functionality later). If you need help on how conda-build's tests work, don't hesitate to ask.

[evhub]

@msarahan Test added and passing!

[msarahan]

Cool, looks good enough for now. I may rewrite it later to be more of a unit test than an integration test, but this is certainly good for now. Thanks!

[github-actions]

Hi there, thank you for your contribution!

This pull request has been automatically locked because it has not had recent activity after being closed.

Please open a new issue or pull request if needed.

Thanks!