🐛 FIX: Check during doctree-resolve if env has attribute

[najuzilu]

This PR fixes doctree-resolve in the event that the extension is imported but not used.

[codecov-io]

Codecov Report

Merging #16 into master will decrease coverage by 0.27%.
The diff coverage is 50.00%.

@@            Coverage Diff             @@
##           master      #16      +/-   ##
==========================================
- Coverage   96.80%   96.53%   -0.28%     
==========================================
  Files           3        3              
  Lines         344      346       +2     
==========================================
+ Hits          333      334       +1     
- Misses         11       12       +1     

Flag	Coverage Δ
#pytests	96.53% <50.00%> (-0.28%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
sphinx_exercise/__init__.py	94.85% <50.00%> (-0.43%)	⬇️

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