Fixed deprecation warning

[pietroferraiuolo]

Fixed a DeprecationWarning on line 251 of arte/types/mask.py:

.../arte/types/mask.py:251: FutureWarning: `RegionProperties.major_axis_length` is deprecated starting in version 0.26 and will be removed in version 2.0. Use `RegionProperties.axis_major_length` instead. 
  r = bubble.major_axis_length / 2.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 76.33%. Comparing base (cc7c4ca) to head (2952192).
⚠️ Report is 14 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #69      +/-   ##
==========================================
- Coverage   76.33%   76.33%   -0.01%     
==========================================
  Files         185      185              
  Lines       12658    12662       +4     
==========================================
+ Hits         9662     9665       +3     
- Misses       2996     2997       +1     

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