FIX: Check for sphinx_exercise_registry in all registered post_tranforms

[mmcky]

This fixes #44

[codecov-commenter]

Codecov Report

Merging #47 (ceb0489) into master (9715a72) will decrease coverage by 0.39%.
The diff coverage is 50.00%.

@@            Coverage Diff             @@
##           master      #47      +/-   ##
==========================================
- Coverage   93.34%   92.95%   -0.40%     
==========================================
  Files           6        6              
  Lines         436      440       +4     
==========================================
+ Hits          407      409       +2     
- Misses         29       31       +2     

Flag	Coverage Δ
pytests	92.95% <50.00%> (-0.40%)	⬇️

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Impacted Files	Coverage Δ
sphinx_exercise/post_transforms.py	93.70% <50.00%> (-1.26%)	⬇️

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