GithubObjects are no longer serializable with pickle

[josh-fell]

To support data exchange, being able to serialize non-JSON-serializable GithubObjects has been easily achieved in past with pickle. However, starting with v1.58, GithubObjects like Repository, NamedUser, etc. are no longer serializable with with pickle,

For example,

import github
import pickle

repo = github.Github().get_repo(full_name_or_id="PyGithub/PyGithub")
pickle.dumps(repo)

raise the following exception:

Traceback (most recent call last):
  File "/Users/joshfell/code/daedalus/tempCodeRunnerFile.python", line 15, in <module>
    pickle.dumps(repo)
  File "/usr/local/Cellar/python@3.10/3.10.9/Frameworks/Python.framework/Versions/3.10/lib/python3.10/multiprocessing/synchronize.py", line 101, in __getstate__
    context.assert_spawning(self)
  File "/usr/local/Cellar/python@3.10/3.10.9/Frameworks/Python.framework/Versions/3.10/lib/python3.10/multiprocessing/context.py", line 373, in assert_spawning
    raise RuntimeError(
RuntimeError: RLock objects should only be shared between processes through inheritance

The underyling Requester object now has an RLock() attribute, __auth_lock, which looks to be preventing this serialization.

[EnricoMi]

Pickle allows Requester to exclude the lock from being serialized:
https://docs.python.org/3/library/pickle.html#handling-stateful-objects

And we should add tests that prove pickle support.

[josh-fell]

@EnricoMi, Thanks for taking a look so quickly and the tip on the pickle workaround!