Create marker for internal tests (#8299)

To help avoid issues with downstream packagers etc. we should disable a
few tests for them which are dependent on the CI environment.

CHANGES/8299.packaging.rst

@@ -0,0 +1,2 @@
+Added an ``internal`` pytest marker for tests which should be skipped
+by packagers (use ``-m 'not internal'`` to disable them) -- by :user:`Dreamsorcerer`.

setup.cfg

@@ -170,3 +170,4 @@ junit_family=xunit2
 xfail_strict = true
 markers =
     dev_mode: mark test to run in dev mode.
+    internal: tests which may cause issues for packagers, but should be run in aiohttp's CI.

tests/test_imports.py

@@ -33,6 +33,7 @@ def test_web___all__(pytester: pytest.Pytester) -> None:
 }
 
 
+@pytest.mark.internal
 @pytest.mark.skipif(
     not sys.platform.startswith("linux") or platform.python_implementation() == "PyPy",
     reason="Timing is more reliable on Linux",