If no tests then Pytest should exit with 0 (not 5)

- Add a 'test' target to run pytest against the 'tests' directory. If an
  exit code of 5 is found then exit with a 0 instead. Otherwise exit
  with the appropriate error code.

- Intercept 'pytest_sessionfinish' to do the same thing in conftest.py
  [1].

- Switch off 'disallowed_untyped_defs' in mypy.ini as hints should help
  readability where it makes sense (hence 'hints'). It should not be enforced
  all the time IMHO.

[1] pytest-dev/pytest#2393

.github/workflows/python-app.yml

@@ -34,4 +34,4 @@ jobs:
       - name: Check types
         run: make types
       - name: Run tests
-        run: pytest -o console_output_style=classic -v
+        run: pytest -o console_output_style=classic -v || ([ $? = 5 ] && exit 0 || exit $?)

Makefile

@@ -26,3 +26,7 @@ format:
 .PHONY: types
 types:
 	mypy .
+
+.PHONY: test
+test:
+	sh -c 'pytest tests || ([ $$? = 5 ] && exit 0 || exit $$?)'

test

@@ -3,4 +3,4 @@
 set -e
 
 make check
-pytest "$@"
+make test

tests/conftest.py

@@ -0,0 +1,7 @@
+NO_TESTS_COLLECTED = 5
+SUCCESS = 0
+
+
+def pytest_sessionfinish(session, exitstatus):
+    if exitstatus == NO_TESTS_COLLECTED and session.config.getoption("-k"):
+        session.exitstatus = SUCCESS