typing: set warn_unreachable

This makes mypy raise an error whenever it detects code which is
statically unreachable, e.g.

    x: int
    if isinstance(x, str):
        ... # Statement is unreachable  [unreachable]

This is really neat and finds quite a few logic and typing bugs.

Sometimes the code is intentionally unreachable in terms of types, e.g.
raising TypeError when a function is given an argument with a wrong
type. In these cases a `type: ignore[unreachable]` is needed, but I
think it's a nice code hint.

testing/code/test_source.py

@@ -238,7 +238,7 @@ def test_getline_finally() -> None:
             c(1)  # type: ignore
         finally:
             if teardown:
-                teardown()
+                teardown()  # type: ignore[unreachable]
     source = excinfo.traceback[-1].statement
     assert str(source).strip() == "c(1)  # type: ignore"