fix ms gate equality

[richrines1]

fixes #6230 by adding a value_equality decorator to XXPowGate, YYPowGate and ZZPowGate in order to override the distinct_child_types=True configuration they inherit from EigenGate

this mimics what is already done for cirq.XPowGate, cirq.ZPowGate, and cirq.ZPowGate

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: -0.01% ⚠️

Comparison is base (14baaa5) 97.37% compared to head (5e14535) 97.37%.
Report is 1 commits behind head on master.

❗ Current head 5e14535 differs from pull request most recent head 27e0071. Consider uploading reports for the commit 27e0071 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6231      +/-   ##
==========================================
- Coverage   97.37%   97.37%   -0.01%     
==========================================
  Files        1116     1116              
  Lines       96042    96054      +12     
==========================================
+ Hits        93524    93532       +8     
- Misses       2518     2522       +4     

Files Changed	Coverage Δ
cirq-core/cirq/ops/parity_gates.py	98.15% <100.00%> (+0.03%)	⬆️
cirq-core/cirq/ops/parity_gates_test.py	100.00% <100.00%> (ø)

... and 5 files with indirect coverage changes

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[vtomole]

LGTM