Fix ipnetwork __contains__ on Python 3.14

[yunzheng]

fixes #193

[yunzheng]

Removes the _is_subnet_of function which was copied because it was not available on older Python versions. Now we can just use IPNetwork.subnet_of as we dropped support for older Python versions.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.42%. Comparing base (e5a9f56) to head (22676ae).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #194      +/-   ##
==========================================
+ Coverage   83.38%   83.42%   +0.04%     
==========================================
  Files          35       35              
  Lines        3707     3699       -8     
==========================================
- Hits         3091     3086       -5     
+ Misses        616      613       -3     

Flag	Coverage Δ
unittests	83.42% <100.00%> (+0.04%)	⬆️

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