Speedup deepcopy for Rating objects

[vivekjoshy]

Affirmation

By submitting this Pull Request or typing my (user)name below,
I affirm the Developer Certificate of Origin
with respect to all commits and content included in this PR,
and understand I am releasing the same under openskill.py's MIT license.

I certify the above statement is true and correct: @vivekjoshy

[codecov]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (a03847b) 100.00% compared to head (2e60ba9) 100.00%.

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #108   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           10        10           
  Lines         1754      1775   +21     
  Branches       450       450           
=========================================
+ Hits          1754      1775   +21     

Files Changed	Coverage Δ
openskill/models/weng_lin/bradley_terry_full.py	100.00% <100.00%> (ø)
openskill/models/weng_lin/bradley_terry_part.py	100.00% <100.00%> (ø)
openskill/models/weng_lin/plackett_luce.py	100.00% <100.00%> (ø)
...nskill/models/weng_lin/thurstone_mosteller_full.py	100.00% <100.00%> (ø)
...nskill/models/weng_lin/thurstone_mosteller_part.py	100.00% <100.00%> (ø)

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