Allow int dtypes in mean (and std, var)

[tomwhite]

Loosen from just floating dtypes, as ints are often passed to mean in practice.

From https://data-apis.org/array-api/latest/API_specification/generated/array_api.mean.html#array_api.mean:

While this specification recommends that this function only accept input arrays having a floating-point data type, specification-compliant array libraries may choose to accept input arrays having an integer data type. While mixed data type promotion is implementation-defined, if the input array x has an integer data type, the returned array must have the default real-valued floating-point data type.