Remove support for python 3.6 and 3.7 from pyproject.toml?

[phargogh]

Our current revision of pyproject.toml supports all versions of python > 3.6. In terms of language features, this is correct. However, should we keep supporting versions of python that are EOL? Maybe not. It would be good to review the PyPI download stats for these versions from the last few months and see just how much traffic python 3.6 and 3.7 are getting, and then use that information (and anything else relevant) to make a decision about whether to support these two EOL python versions.