Update SupportsIndex support for math and cmath (#6216)

* SupportsIndex for cmath
* Don't support __index__ on <3.7

stdlib/cmath.pyi

@@ -1,5 +1,9 @@
+import sys
 from typing import SupportsComplex, SupportsFloat, Union
 
+if sys.version_info >= (3, 8):
+    from typing import SupportsIndex
+
 e: float
 pi: float
 inf: float
@@ -8,7 +12,10 @@ nan: float
 nanj: complex
 tau: float
 
-_C = Union[SupportsFloat, SupportsComplex, complex]
+if sys.version_info >= (3, 8):
+    _C = Union[SupportsFloat, SupportsComplex, SupportsIndex, complex]
+else:
+    _C = Union[SupportsFloat, SupportsComplex, complex]
 
 def acos(__z: _C) -> complex: ...
 def acosh(__z: _C) -> complex: ...

stdlib/math.pyi

@@ -3,7 +3,10 @@ from _typeshed import SupportsTrunc
 from typing import Iterable, SupportsFloat, Union, overload
 from typing_extensions import SupportsIndex
 
-_SupportsFloatOrIndex = Union[SupportsFloat, SupportsIndex]
+if sys.version_info >= (3, 8):
+    _SupportsFloatOrIndex = Union[SupportsFloat, SupportsIndex]
+else:
+    _SupportsFloatOrIndex = SupportsFloat
 
 e: float
 pi: float
@@ -36,7 +39,13 @@ def erfc(__x: _SupportsFloatOrIndex) -> float: ...
 def exp(__x: _SupportsFloatOrIndex) -> float: ...
 def expm1(__x: _SupportsFloatOrIndex) -> float: ...
 def fabs(__x: _SupportsFloatOrIndex) -> float: ...
-def factorial(__x: SupportsIndex) -> int: ...
+
+if sys.version_info >= (3, 8):
+    def factorial(__x: SupportsIndex) -> int: ...
+
+else:
+    def factorial(__x: int) -> int: ...
+
 def floor(__x: _SupportsFloatOrIndex) -> int: ...
 def fmod(__x: _SupportsFloatOrIndex, __y: _SupportsFloatOrIndex) -> float: ...
 def frexp(__x: _SupportsFloatOrIndex) -> tuple[float, int]: ...