use unittest.mock instead of mock

[pgajdos]

Could you please consider to use unittest.mock at least as fallback, when mock is not installed?

[hugovk]

Please see PR #536 that drops support for EOL Python and replaces the mock library with the stdlib.

[pgajdos]

Thanks!

[eli-schwartz]

Interesting that this ticket was closed without a PR merged to implement it. :)