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.

src/_pytest/python_api.py

@@ -495,7 +495,8 @@ def approx(expected, rel=None, abs=None, nan_ok: bool = False) -> ApproxBase:
     elif (
         isinstance(expected, Iterable)
         and isinstance(expected, Sized)
-        and not isinstance(expected, STRING_TYPES)
+        # Type ignored because the error is wrong -- not unreachable.
+        and not isinstance(expected, STRING_TYPES)  # type: ignore[unreachable]
     ):
         cls = ApproxSequencelike
     else:
@@ -662,8 +663,8 @@ def raises(  # noqa: F811
     else:
         excepted_exceptions = expected_exception
     for exc in excepted_exceptions:
-        if not isinstance(exc, type) or not issubclass(exc, BaseException):
-            msg = "expected exception must be a BaseException type, not {}"
+        if not isinstance(exc, type) or not issubclass(exc, BaseException):  # type: ignore[unreachable]
+            msg = "expected exception must be a BaseException type, not {}"  # type: ignore[unreachable]
             not_a = exc.__name__ if isinstance(exc, type) else type(exc).__name__
             raise TypeError(msg.format(not_a))