Add test for proper flake8 invocation of the checker

Add flake8 to the tox environment due to added subprocess call

testing/test_flake8_actually_runs_checker.py

@@ -0,0 +1,9 @@
+from subprocess import PIPE, run
+
+
+def test_checker_runs() -> None:
+    """Test that the checker is properly registered by Flake8 as needing to run on the input src."""
+    substr = "TYP001 Missing type annotation for function argument 'x'"
+    p = run(["flake8", "-"], stdout=PIPE, input="def bar(x) -> None:\n    pass\n", encoding="ascii")
+
+    assert substr in p.stdout

tox.ini

@@ -41,6 +41,7 @@ deps =
     pytest
     pytest-check
     pytest-cov
+    flake8
 
 [testenv:clean]
 deps = coverage