use unittest.mock instead of mock

[pgajdos]

Could you please consider to utilize unittest.mock instead of mock?

Background:
https://fedoraproject.org/wiki/Changes/DeprecatePythonMock
https://trello.com/c/S6eADbii/64-remove-python-mock

[pgajdos]

At least 3.8, 3.9 and 3.10 testsuite works with that:
https://build.opensuse.org/package/show/home:pgajdos:python/python-graphene-django/

[Ülgen Sarıkavak]

@pgajdos It seems this is resolved by #1336 If you think the problem still exists, feel free to reopen this issue.