Use TypeAlias where possible for type aliases (#7630)

stdlib/math.pyi

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