Tighten annotation of logging.getLevelName (#12088)

To better reflect the implementation's behaviour,
#2730 changed
`logging.getLevelName` to accept `int | str` and return `Any` (the
latter due to the need to avoid union return types).  However, this
isn't ideal if you're passing in an `int`, in which case the
implementation always returns a `str`.  Add overloads for this.

Author: cjwatson

stdlib/logging/__init__.pyi

@@ -572,7 +572,14 @@ fatal = critical
 
 def disable(level: int = 50) -> None: ...
 def addLevelName(level: int, levelName: str) -> None: ...
-def getLevelName(level: _Level) -> Any: ...
+@overload
+def getLevelName(level: int) -> str: ...
+
+# The str -> int case is considered a mistake, but retained for backward
+# compatibility.  See
+# https://docs.python.org/3/library/logging.html#logging.getLevelName.
+@overload
+def getLevelName(level: str) -> Any: ...
 
 if sys.version_info >= (3, 11):
     def getLevelNamesMapping() -> dict[str, int]: ...