Let unittest frameworks deal with async functions

Instead of trying to handle unittest-async functions in pytest_pyfunc_call,
let the unittest framework handle them instead.

This lets us remove the hack in pytest_pyfunc_call, with the upside that
we should support any unittest-async based framework.

Also included 'asynctest' as test dependency for py37-twisted, and renamed
'twisted' to 'unittestextras' to better reflect that we install 'twisted' and
'asynctest' now.

This also fixes the problem of cleanUp functions not being properly called
for async functions.

Fix #7110
Fix #6924

src/_pytest/python.py

@@ -34,8 +34,8 @@ from _pytest.compat import get_default_arg_names
 from _pytest.compat import get_real_func
 from _pytest.compat import getimfunc
 from _pytest.compat import getlocation
+from _pytest.compat import is_async_function
 from _pytest.compat import is_generator
-from _pytest.compat import iscoroutinefunction
 from _pytest.compat import NOTSET
 from _pytest.compat import REGEX_TYPE
 from _pytest.compat import safe_getattr
@@ -159,7 +159,7 @@ def pytest_configure(config):
     )
 
 
-def async_warn(nodeid: str) -> None:
+def async_warn_and_skip(nodeid: str) -> None:
     msg = "async def functions are not natively supported and have been skipped.\n"
     msg += (
         "You need to install a suitable plugin for your async framework, for example:\n"
@@ -175,33 +175,13 @@ def async_warn(nodeid: str) -> None:
 @hookimpl(trylast=True)
 def pytest_pyfunc_call(pyfuncitem: "Function"):
     testfunction = pyfuncitem.obj
-
-    try:
-        # ignoring type as the import is  invalid in py37  and mypy thinks its a error
-        from unittest import IsolatedAsyncioTestCase  # type: ignore
-    except ImportError:
-        async_ok_in_stdlib = False
-    else:
-        async_ok_in_stdlib = isinstance(
-            getattr(testfunction, "__self__", None), IsolatedAsyncioTestCase
-        )
-
-    if (
-        iscoroutinefunction(testfunction)
-        or (sys.version_info >= (3, 6) and inspect.isasyncgenfunction(testfunction))
-    ) and not async_ok_in_stdlib:
-        async_warn(pyfuncitem.nodeid)
+    if is_async_function(testfunction):
+        async_warn_and_skip(pyfuncitem.nodeid)
     funcargs = pyfuncitem.funcargs
     testargs = {arg: funcargs[arg] for arg in pyfuncitem._fixtureinfo.argnames}
     result = testfunction(**testargs)
     if hasattr(result, "__await__") or hasattr(result, "__aiter__"):
-        if async_ok_in_stdlib:
-            # todo: investigate moving this to the unittest plugin
-            #       by a test call result hook
-            testcase = testfunction.__self__
-            testcase._callMaybeAsync(lambda: result)
-        else:
-            async_warn(pyfuncitem.nodeid)
+        async_warn_and_skip(pyfuncitem.nodeid)
     return True