Migrate asynctest to stdlib's AsyncMock

[rathann]

Problem Description

test/mitmproxy/addons/test_readfile.py:5 imports asynctest which doesn't work with python 3.8 and above.

asynctest upstream is aware of this breakage and recommends using the built-in AsyncMock instead. See also asynctest/issues/144 and asynctest/issues/126 .

Fortunately, this is the only test that seems to be affected.

[Kriechi]

We do have a green CI test on Python 3.8:
https://github.com/mitmproxy/mitmproxy/runs/724843736?check_suite_focus=true

How is the test failing for you?

[mhils]

We probably want to migrate to the stdlib implementation here, but that requires us to drop Python 3.7 and below. I'd be fine doing this, but we're blocked by PyInstaller which does not support 3.8 yet (pyinstaller/pyinstaller#4311).

[bnavigator]

You are using pytest-asyncio already. Replace the asynctest.patch calls with fixture from pytest-mock? pytest-dev/pytest-asyncio#42

[mhils]

@bnavigator: works for me as well. If you would like to send a PR, that would be fabulous! 😃