Fix mechanism to detect inherited static fields

[cgarciae]

Fixes #8

• Change mechanism to detect inherited static fields + some slight optimizations.
• Adds example from Inheritance of dataclasses #8 as a test.
• Use frozenset instead of set for safety on various metadata fields.

[codecov]

Codecov Report

Merging #9 (5c2314a) into main (8419d6f) will increase coverage by 1.45%.
The diff coverage is 98.71%.

@@            Coverage Diff             @@
##             main       #9      +/-   ##
==========================================
+ Coverage   96.00%   97.45%   +1.45%     
==========================================
  Files           2        2              
  Lines         100      118      +18     
==========================================
+ Hits           96      115      +19     
+ Misses          4        3       -1     

Impacted Files	Coverage Δ
simple_pytree/pytree.py	97.39% <98.70%> (+1.47%)	⬆️
simple_pytree/__init__.py	100.00% <100.00%> (ø)

📣 We’re building smart automated test selection to slash your CI/CD build times. Learn more