Bump pytest to version 6.0.1 and yarl to 1.5.1 (#4126)

* Update pytest to version 6.0.1

* remove some awful monkeypatching

* use new import mode

* Update yarl to 1.5.1

redbot/pytest/core.py

@@ -4,13 +4,11 @@ from pathlib import Path
 import weakref
 
 import pytest
-from _pytest.monkeypatch import MonkeyPatch
 from redbot.core import Config
 from redbot.core.bot import Red
 from redbot.core import config as config_module, drivers
 
 __all__ = [
-    "monkeysession",
     "override_data_path",
     "coroutine",
     "driver",
@@ -30,13 +28,6 @@ __all__ = [
 ]
 
 
-@pytest.fixture(scope="session")
-def monkeysession(request):
-    mpatch = MonkeyPatch()
-    yield mpatch
-    mpatch.undo()
-
-
 @pytest.fixture(autouse=True)
 def override_data_path(tmpdir):
     from redbot.core import data_manager