Add support for the 'visibility' attribute on the Repository object

[lightningboltemoji]

Adds support for the 'visibility' attribute on the Repository object. Closes #1446.

I updated the affected replay tests manually (not seeing a BeaverSoftware org?). I added the header to reflect the new request we'll send, and added "visibility":"public" next to "private":false in all response objects.

I also smoke tested this against our real enterprise account:

$ python3
Python 3.9.1 (default, Jan 30 2021, 15:51:05) 
[Clang 12.0.0 (clang-1200.0.32.29)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import github
>>> g = github.Github("<token>")
>>> repos = list(g.get_organization("my-enterprise").get_repos())
>>> repos[0].private
False
>>> repos[0].visibility
'public'
>>> repos[10].private
True
>>> repos[10].visibility
'internal'
>>> g.get_organization("my-enterprise").get_repo("my-repo").private
True
>>> g.get_organization("my-enterprise").get_repo("my-repo").visibility
'internal'
>>> g.get_organization("my-enterprise").create_repo(name="quick-test", visibility="internal")
Repository(full_name="my-enterprise/quick-test")
# verified this is internal
>>> g.get_organization("my-enterprise").create_repo(name="quick-test-2", private=True)
Repository(full_name="my-enterprise/quick-test-2")
# verified this is private

Thank you for the useful library and taking the time to consider this contribution!

[codecov-io]

Codecov Report

Merging #1872 (75fff25) into master (aa24030) will decrease coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1872      +/-   ##
==========================================
- Coverage   98.87%   98.86%   -0.02%     
==========================================
  Files         108      108              
  Lines       11044    11055      +11     
==========================================
+ Hits        10920    10929       +9     
- Misses        124      126       +2     

Impacted Files	Coverage Δ
...runner/work/PyGithub/PyGithub/github/Repository.py	97.28% <0.00%> (-0.12%)	⬇️
...ome/runner/work/PyGithub/PyGithub/github/Consts.py	100.00% <0.00%> (ø)
...nner/work/PyGithub/PyGithub/github/Organization.py	98.56% <0.00%> (+<0.01%)	⬆️

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[s-t-e-v-e-n-k]

Rebase on master, please -- I've deleted those files that are conflicting.

[lightningboltemoji]

Hi @s-t-e-v-e-n-k, I've rebased on top of master

[sfdc-afraley]

Any ETA on getting this merged and released?

[codecov-commenter]

Codecov Report

Merging #1872 (e12d559) into master (b10dbe3) will decrease coverage by 0.01%.
The diff coverage is n/a.

❗ Current head e12d559 differs from pull request most recent head 178e23d. Consider uploading reports for the commit 178e23d to get more accurate results

@@            Coverage Diff             @@
##           master    #1872      +/-   ##
==========================================
- Coverage   98.86%   98.84%   -0.02%     
==========================================
  Files         108      108              
  Lines       11064    11071       +7     
==========================================
+ Hits        10938    10943       +5     
- Misses        126      128       +2     

Impacted Files	Coverage Δ
github/PullRequestComment.py
github/StatsContributor.py
github/Comparison.py
github/Branch.py
github/GitCommit.py
github/Stargazer.py
github/SourceImport.py
github/GitTree.py
github/Authorization.py
github/TeamDiscussion.py
... and 203 more

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[s-t-e-v-e-n-k]

Does this also require changes to MainClass.get_repo to add in the Accept header?

[s-t-e-v-e-n-k]

Does this also require changes to MainClass.get_repo to add in the Accept header?

It does not, I have checked.