This is a continuation of the work started by @allevin on https://github.com/PyGithub/PyGithub/pull/860.
I have refactored the testing Framework to use the `httpretty` library in order to use the urllib3 retry feature and therefore be able to test it.
Please refer to https://github.com/PyGithub/PyGithub/pull/860 for additional context.
cc: @allevin @mfonville @jrouquie @sfdye
Closes#757Closes#860
