Implement GithubRetry DEFAULT_ALLOWED_METHODS and DEFAULT_STATUS_FORCELIST

[jodelasur]

GithubRetry's default values for status_forcelist and allowed_methods are computed in __init__. See here. We had a use case where we wanted extend (not override) the default values, but we resorted to copy-pasting. It would be better to have class members for it, similar to Retry.DEFAULT_ALLOWED_METHODS.

Instead of:

GithubRetry(
    # GithubRetry default value + 401
    status_forcelist=list(range(500, 600)) + [403, 401],
    # GithubRetry default value + "PATCH". 
    # GithubRetry.DEFAULT_ALLOWED_METHODS currently inherited from urllib3.Retry
    allowed_methods=GithubRetry.DEFAULT_ALLOWED_METHODS.union(
        {"GET", "POST", "PATCH"}
    ),
)

it would be better to have:

GithubRetry(
    status_forcelist=GithubRetry.DEFAULT_STATUS_FORCELIST + [401],
    allowed_methods=GithubRetry.DEFAULT_ALLOWED_METHODS | {"PATCH"},
)

[EnricoMi]

This sounds useful, please go ahead and create a pull request!