[Python-ideas] Add gamma function to cmath module

Since the gamma function (see https://en.wikipedia.org/wiki/Gamma_function) is defined for complex numbers too, wouldn't it be normal to have it implemented on the `cmath` module? The `math` module has this function, but it doesn't support complex numbers (as expected). I know that others libraries such as scipy (on the `scipy.special` module), or the `mpmath` module already have what I'm requesting but since `cmath` module is the complementary of `math`. I think that if others functions such as `sin`, `cos`, `sqrt` are implemented on both modules, why shouldn't gamma function?