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/pathlib.py

@@ -367,7 +367,6 @@ def make_numbered_dir_with_cleanup(
 
 
 def resolve_from_str(input: str, root: py.path.local) -> Path:
-    assert not isinstance(input, Path), "would break on py2"
     rootpath = Path(root)
     input = expanduser(input)
     input = expandvars(input)