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/doctest.py

@@ -635,8 +635,8 @@ def _init_checker_class() -> "Type[doctest.OutputChecker]":
                 return got
             offset = 0
             for w, g in zip(wants, gots):
-                fraction = w.group("fraction")
-                exponent = w.group("exponent1")
+                fraction = w.group("fraction")  # type: Optional[str]
+                exponent = w.group("exponent1")  # type: Optional[str]
                 if exponent is None:
                     exponent = w.group("exponent2")
                 if fraction is None: