Add internal as valid Repository visibility value

[AndrewJDawes]

Issue:

• PyGithub currently throws an AssertionError when attempting to create or edit a Repository where param visibility="internal".
• This is an issue because "internal" is a valid Repository visibility value for GitHub Enterprise users.
• https://docs.github.com/en/enterprise-cloud@latest/rest/repos/repos?apiVersion=2022-11-28#create-an-organization-repository

Example of the issue:

org = gh.get_organization(organization)
org.create_repo(name="test-repo", visibility="internal")
# throws AssertionError and fails to create repo

Proposed fix (in pull request):

• Allow "internal" as a valid value for visibility

Example:

org = gh.get_organization(organization)
org.create_repo(name="test-repo", visibility="internal")
# does not throw assertion error and successfully creates repo

[AndrewJDawes]

Fixes #2810

[EnricoMi]

LGTM!

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (d92cfba) 96.73% compared to head (4841ef7) 96.73%.

❗ Your organization needs to install the Codecov GitHub app to enable full functionality.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2806   +/-   ##
=======================================
  Coverage   96.73%   96.73%           
=======================================
  Files         139      139           
  Lines       14192    14192           
=======================================
  Hits        13729    13729           
  Misses        463      463           

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