Add complex number support to sum

[kgryte]

This PR

• adds support for complex number summation. As described in Add support for complex number addition #525, complex number addition is well-defined, where (a + bj) + (c + dj) = (a+c) + (c+d)j.
• specifies that special cases be derived as if the operation was implemented as a reduction over add.
• requires that, if x is a complex floating-point data type and dtype is None, the function must return an array having the default complex floating-point data type.
• updates the input and output array data types to be any numeric data type, not just real-valued data types.
• depends on Add support for complex number addition #525.

[rgommers]

Nice that the special cases compose. This LGTM, thanks @kgryte