Verify that JSONDecodeError is an actual subclass of ValueError, not just an alias

[JustAnotherArchivist]

Follow-up for #498. Sorry, I forgot that issubclass(x, x) is True, therefore that test would pass even if JSONDecodeError were simply an alias of ValueError rather than a proper subclass. This adds a check that they are not identical.

[codecov-commenter]

Codecov Report

Merging #500 (4ffc7dd) into main (316d384) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #500   +/-   ##
=======================================
  Coverage   88.78%   88.79%           
=======================================
  Files           6        6           
  Lines        1694     1695    +1     
=======================================
+ Hits         1504     1505    +1     
  Misses        190      190           

Impacted Files	Coverage Δ
tests/test_ujson.py	99.54% <100.00%> (+<0.01%)	⬆️

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[hugovk]

Thanks!