Type hint for _RequestHistoryTracker.reset method

[lhelwerd]

This avoids mypy errors in typed code that follows the last example at https://requests-mock.readthedocs.io/en/latest/history.html#reset-history

[jamielennox]

thank you.

I keep having things like this, now that we don't support python2 any more i should merge the type hints because we keep missing little things like this.