feat: add allow_forking to Repository

[IbrahimAH]

Closes #2310.

based on #2310
but up to date and with passing tests

Adds to the allow_forking attribute to a Repository: https://docs.github.com/en/rest/repos/repos#update-a-repository

allow_forking boolean
Either true to allow private forks, or false to prevent private forks.

This attribute can be modified via edit (on a private repository):

>>> g.get_repo("<test repo>").allow_forking
True
>>> g.get_repo("<test repo>").edit(allow_forking=False)
>>> g.get_repo("<test repo>").allow_forking
False
>>> g.get_repo("<test repo>").edit(allow_forking=True)
>>> g.get_repo("<test repo>").allow_forking
True

[codecov-commenter]

Codecov Report

Base: 98.75% // Head: 98.75% // Increases project coverage by +0.00% 🎉

Coverage data is based on head (b3f7aaf) compared to base (7902351).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2380   +/-   ##
=======================================
  Coverage   98.75%   98.75%           
=======================================
  Files         115      115           
  Lines       11578    11588   +10     
=======================================
+ Hits        11434    11444   +10     
  Misses        144      144           

Impacted Files	Coverage Δ
github/Repository.py	97.20% <100.00%> (+0.01%)	⬆️

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[IbrahimAH]

@sfdye sorry for the direct tag, was hoping you'd be able to review this as the original PR this was based off became outdated after a few months of not being merged in. Thank you