Add __module__ attributes to exception classes.

[methane]

Fixes #522.

[codecov]

Codecov Report

Merging #523 (0058bb9) into main (5340191) will increase coverage by 0.28%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #523      +/-   ##
==========================================
+ Coverage   85.71%   86.00%   +0.28%     
==========================================
  Files           6        6              
  Lines         539      550      +11     
==========================================
+ Hits          462      473      +11     
  Misses         77       77              

Impacted Files	Coverage Δ
MySQLdb/_exceptions.py	100.00% <100.00%> (ø)

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