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

testing/example_scripts/unittest/test_unittest_asynctest.py

@@ -0,0 +1,22 @@
+"""Issue #7110"""
+import asyncio
+
+import asynctest
+
+
+teardowns = []
+
+
+class Test(asynctest.TestCase):
+    async def tearDown(self):
+        teardowns.append(None)
+
+    async def test_error(self):
+        await asyncio.sleep(0)
+        self.fail("failing on purpose")
+
+    async def test_ok(self):
+        await asyncio.sleep(0)
+
+    def test_teardowns(self):
+        assert len(teardowns) == 2