Replace tmpdir fixture with tmp_path in tests

[asvetlov]

I always stumble on py.path methods.
A new pytest supports tmp_path fixture https://docs.pytest.org/en/latest/tmpdir.html#the-tmp-path-factory-fixture to work with pathlib.Path.

The issue is easy and waits for a champion.

[aio-libs-bot]

GitMate.io thinks possibly related issues are #1459 (Tests for demo polls app fail with "fixture 'event_loop' not found"), #2148 (Failed tests), #304 (multipart tests), #858 (replace http parser?), and #1583 (normalize_path_middleware tests failures).

[gyermolenko]

I found it relevant to current issue, adding for context: ref pytest-dev/pytest#4680 and two other tmp_path defects were fixed in pytest 4.2.
And pytest 4.2 has just beed released.