Support Python 3.7-3.9 more officially

[john-science]

Up until now, this project has only officially supported up to Python 3.6 on PyPI, because that is explicitly what is stated in the setup.py. However, I have some evidence:

• In this repo, TOX has already been running unit tests for 3.7 and 3.8.
• In this repo, Travis CI has already been testing Python 3.7 and 3.8.
• The ARMI team has been using Yamlize for Python 3.7 and 3.9 for years.
• Yamlize works fine on my laptop (Ubuntu 20.04) in Python 3.7 and 3.9.

NOTE: I had to rev the version number of yamlize for this to take effect. Essentially, PyPI does not like it if you try to upload a version of a package that already exists. So, in order to get official support for Python 3.7-3.9, yamlize will have to be uploaded to PyPI again with a (very slightly) increased version number.

[john-science]

Bump.

Can we get a new PyPI release with this change and Nick's last change? I know Nick's last change looked small, but it fixes hundreds of errors in our Sphinx process. And we are working hard to get our docs to a production-ready state.

Thanks!

[SimplyKnownAsG]

Done. Thanks!

https://pypi.org/project/yamlize/#files