Require __pow__ and __truediv__ to have floating-point inputs

Previously it was any numeric dtype, but this was inconsistent with the
functions pow() and divide(). Integer inputs are problematic because they
would either always have to be cast to float or use value-based casting
depending on, e.g., the sign of the exponent for __pow__. Furthermore, in
order to properly correspond to Python conventions, __truediv__ should always
return a float (integer division should be implemented by __floordiv__).

This was discussed in data-apis#173.

Author: asmeurer

spec/API_specification/array_object.md

@@ -780,7 +780,7 @@ The `matmul` function must implement the same semantics as the built-in `@` oper
 
 -   **self**: _<array>_
 
-    -   array instance. Should have a numeric data type. Must have at least one dimension. If `self` is one-dimensional having shape `(M)` and `other` has more than one dimension, `self` must be promoted to a two-dimensional array by prepending `1` to its dimensions (i.e., must have shape `(1, M)`). After matrix multiplication, the prepended dimensions in the returned array must be removed. If `self` has more than one dimension (including after vector-to-matrix promotion), `self` must be compatible with `other` (see {ref}`broadcasting`). If `self` has shape `(..., M, K)`, the innermost two dimensions form matrices on which to perform matrix multiplication. 
+    -   array instance. Should have a numeric data type. Must have at least one dimension. If `self` is one-dimensional having shape `(M)` and `other` has more than one dimension, `self` must be promoted to a two-dimensional array by prepending `1` to its dimensions (i.e., must have shape `(1, M)`). After matrix multiplication, the prepended dimensions in the returned array must be removed. If `self` has more than one dimension (including after vector-to-matrix promotion), `self` must be compatible with `other` (see {ref}`broadcasting`). If `self` has shape `(..., M, K)`, the innermost two dimensions form matrices on which to perform matrix multiplication.
 
 -   **other**: _<array>_
 
@@ -809,7 +809,7 @@ The `matmul` function must implement the same semantics as the built-in `@` oper
 
 -   if either `self` or `other` is a zero-dimensional array.
 -   if `self` is a one-dimensional array having shape `(N)`, `other` is a one-dimensional array having shape `(M)`, and `N != M`.
--   if `self` is an array having shape `(..., M, K)`, `other` is an array having shape `(..., L, N)`, and `K != L`. 
+-   if `self` is an array having shape `(..., M, K)`, `other` is an array having shape `(..., L, N)`, and `K != L`.
 
 (method-__mod__)=
 ### \_\_mod\_\_(self, other, /)
@@ -1016,11 +1016,11 @@ For floating-point operands, let `self` equal `x1` and `other` equal `x2`.
 
 -   **self**: _<array>_
 
-    -   array instance whose elements correspond to the exponentiation base. Should have a numeric data type.
+    -   array instance whose elements correspond to the exponentiation base. Should have a floating-point data type.
 
 -   **other**: _Union\[ int, float, <array> ]_
 
-    -   other array whose elements correspond to the exponentiation exponent. Must be compatible with `self` (see {ref}`broadcasting`). Should have a numeric data type.
+    -   other array whose elements correspond to the exponentiation exponent. Must be compatible with `self` (see {ref}`broadcasting`). Should have a floating-point data type.
 
 #### Returns
 
@@ -1149,11 +1149,11 @@ For floating-point operands, let `self` equal `x1` and `other` equal `x2`.
 
 -   **self**: _<array>_
 
-    -   array instance. Should have a numeric data type.
+    -   array instance. Should have a floating-point data type.
 
 -   **other**: _Union\[ int, float, <array> ]_
 
-    -   other array. Must be compatible with `self` (see {ref}`broadcasting`). Should have a numeric data type.
+    -   other array. Must be compatible with `self` (see {ref}`broadcasting`). Should have a floating-point data type.
 
 #### Returns