Add Python version check

[jrbourbeau]

This PR adds the Python version to our version mismatch check. I've also updated get_system_info to return a dictionary instead of a list-of-tuples. This is so the "host" and "packages" data structures returned by get_versions are the same, and it it also made test_python_version_mismatch_warning slightly cleaner to write.

cc @mrocklin

Closes dask/dask#6007

[jrbourbeau]

Planning to merge in a few hours if no further comments

[mrocklin]

Thanks @jrbourbeau !