Keep tests working w/ upcoming aiohttp 4.0.0 (#2974)

aiohttp.test_utils.unittest_run_loop was deprecated since aiohttp 3.8
and aiohttp 4 (which isn't a thing quite yet) removes it. To maintain
compatibility with the full range of versions we declare to support,
test_blackd.py will now define a no-op replacement if it can't be
imported.

Also, mypy is painfully slow to use without a cache, let's reenable it.

mypy.ini

@@ -32,9 +32,6 @@ warn_unreachable=True
 disallow_untyped_defs=True
 check_untyped_defs=True
 
-# No incremental mode
-cache_dir=/dev/null
-
 [mypy-black]
 # The following is because of `patch_click()`. Remove when
 # we drop Python 3.6 support.

tests/test_blackd.py

@@ -1,4 +1,5 @@
 import re
+from typing import Any
 from unittest.mock import patch
 
 from click.testing import CliRunner
@@ -8,12 +9,18 @@
 
 try:
     import blackd
-    from aiohttp.test_utils import AioHTTPTestCase, unittest_run_loop
+    from aiohttp.test_utils import AioHTTPTestCase
     from aiohttp import web
+except ImportError as e:
+    raise RuntimeError("Please install Black with the 'd' extra") from e
+
+try:
+    from aiohttp.test_utils import unittest_run_loop
 except ImportError:
-    has_blackd_deps = False
-else:
-    has_blackd_deps = True
+    # unittest_run_loop is unnecessary and a no-op since aiohttp 3.8, and aiohttp 4
+    # removed it. To maintain compatibility we can make our own no-op decorator.
+    def unittest_run_loop(func: Any, *args: Any, **kwargs: Any) -> Any:
+        return func
 
 
 @pytest.mark.blackd