Export more names from `responses` in pytest-responses

[filbranden]

This makes it easier to use the names, since the fixture name responses clashes with the module import.

For example, it is possible to add a type annotation to a pytest module using the responses fixture:

def test_my_request(responses: pytest_responses.RequestsMock) -> None:
    ...

It is also possible to access matchers via pytest_responses.matchers, also assemble a Response object, use registries, etc.

See related getsentry/responses#739 for exporting more under RequestsMock as well, and recommending use of this pytest-responses plug-in for a standard pytest fixture for responses.

Also update to accessing default start/stop/reset via the requests.mock object, and using the @pytest.fixtures decorator since @yield_fixture has been long deprecated.