🐛 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

❌ Patch coverage is 50.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 96.53%. Comparing base (22f8c36) to head (0de1f68).

Files with missing lines	Patch %	Lines
sphinx_exercise/__init__.py	50.00%	1 Missing ⚠️

❌ Your patch status has failed because the patch coverage (50.00%) is below the target coverage (80.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@            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%)	⬇️

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