Improve handling of no-op and invalid downgrade cases

[tetron]

refs #93

[codecov]

Codecov Report

Merging #94 (64b7003) into main (df84c39) will decrease coverage by 1.20%.
The diff coverage is 29.41%.

❗ Current head 64b7003 differs from pull request most recent head 9c0f879. Consider uploading reports for the commit 9c0f879 to get more accurate results

@@            Coverage Diff             @@
##             main      #94      +/-   ##
==========================================
- Coverage   75.51%   74.31%   -1.21%     
==========================================
  Files           4        4              
  Lines         535      545      +10     
==========================================
+ Hits          404      405       +1     
- Misses        131      140       +9     

Impacted Files	Coverage Δ
cwlupgrader/main.py	71.13% <29.41%> (-1.29%)	⬇️

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[tetron]

How the heck does mypy pass on an unbound variable but not pass on a None-assigned variable that is clearly reassigned to non-None on every possible code path?