Clean up old Python 3.6 code paths

[jayqi]

There are a few code paths for handling Python 3.6 typing library quirks. These should no longer be necessary.

Also minor documentation changes:

• Switches mkdocstrings handler back to python-legacy. The new python handler doesn't yet properly handle properties. Properties are rendered with parens mkdocstrings/python#9
• Adds TOC entries for modules so they show up in the Intersphinx inventory file

[pjbull]

just curious about the reason for explicitly including 3.7.3?

[jayqi]

It's because some of the imports are for cases < 3.7.4. Those stopped being covered when we removed 3.6 in #56.

https://app.codecov.io/gh/drivendataorg/erdantic/compare/56

[github-actions]

🚀 Deployed on https://deploy-preview-57--nervous-visvesvaraya-4b7042.netlify.app

[codecov-commenter]

Codecov Report

Merging #57 (dabc381) into main (91beca6) will increase coverage by 2.4%.
The diff coverage is 100.0%.

@@           Coverage Diff           @@
##            main     #57     +/-   ##
=======================================
+ Coverage   95.5%   97.9%   +2.4%     
=======================================
  Files         13      13             
  Lines        517     492     -25     
=======================================
- Hits         494     482     -12     
+ Misses        23      10     -13     

Impacted Files	Coverage Δ
erdantic/exceptions.py	97.2% <100.0%> (+6.5%)	⬆️
erdantic/typing.py	100.0% <100.0%> (+11.7%)	⬆️