Use PEP 604 syntax wherever possible (#7493)

stdlib/math.pyi

@@ -1,10 +1,10 @@
 import sys
 from _typeshed import SupportsTrunc
-from typing import Iterable, SupportsFloat, Union, overload
+from typing import Iterable, SupportsFloat, overload
 from typing_extensions import SupportsIndex
 
 if sys.version_info >= (3, 8):
-    _SupportsFloatOrIndex = Union[SupportsFloat, SupportsIndex]
+    _SupportsFloatOrIndex = SupportsFloat | SupportsIndex
 else:
     _SupportsFloatOrIndex = SupportsFloat