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/tmpdir.py

@@ -26,7 +26,7 @@ class TempPathFactory:
     """
 
     _given_basetemp = attr.ib(
-        type=Path,
+        type=Optional[Path],
         # Use os.path.abspath() to get absolute path instead of resolve() as it
         # does not work the same in all platforms (see #4427).
         # Path.absolute() exists, but it is not public (see https://bugs.python.org/issue25012).