Fix the check for None type in inspect._get_parser

[jdidion]

type(type(None)) returns <class type>, and so isinstance(type_, type(type(None))) always returns True. Instead, we want type_ is type(None). I used @msto suggestion to alias NoneType = type(None) to avoid the flake8 E721 error.

Closes #94.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (94b3696) 93.34% compared to head (990d303) 93.68%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #96      +/-   ##
==========================================
+ Coverage   93.34%   93.68%   +0.34%     
==========================================
  Files          32       32              
  Lines        3364     3374      +10     
  Branches      619      622       +3     
==========================================
+ Hits         3140     3161      +21     
+ Misses        149      140       -9     
+ Partials       75       73       -2     

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[TedBrookings]

The types and inspect modules are quite complex and the fact that this weird edge-case resulted in incorrect behavior suggests to me that many more tests are needed. But that's a separate issue. I think this PR correctly addresses the issue at hand.