[#2454] Add IPv6Address to JSONEncoderMixin

[pi-sigma]

Support for encoding IPv6 addresses was missing from the JSONEncoderMixin used for creating subscriptions

Taiga: #2454

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.20%. Comparing base (c54f8e1) to head (192b482).
Report is 3 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #1202   +/-   ##
========================================
  Coverage    95.20%   95.20%           
========================================
  Files          958      959    +1     
  Lines        34608    34620   +12     
========================================
+ Hits         32949    32961   +12     
  Misses        1659     1659           

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[Bartvaderkin]

Code seems fine but is is unclear if they support IPv6, let's assume they do.

[alextreme]

We'll know soon enough...