Use typing.Literal without TYPE_CHECKING checks

Literal was added in Python 3.8 which we now require so can use it
freely.

src/_pytest/_py/path.py

@@ -25,14 +25,12 @@ from stat import S_ISREG
 from typing import Any
 from typing import Callable
 from typing import cast
+from typing import Literal
 from typing import overload
 from typing import TYPE_CHECKING
 
 from . import error
 
-if TYPE_CHECKING:
-    from typing import Literal
-
 # Moved from local.py.
 iswin32 = sys.platform == "win32" or (getattr(os, "_name", False) == "nt")