Adding complex inputs to `polygamma` functions

[LorenzoPeri17]

Hi! I am a (former) PhD student from Cambirdge. During my PhD I have expanded the algorithms in the current digamma implementation to calculate higher orders of the polygamma function with high precision.

I would be happy to make a PR and contribute them to xsf if this would be welcome! :)

It would add functionality and close this long-standing issiue in scipy about complex polygammas lacking in scipy.special.

[steppi]

Thanks @LorenzoPeri17! A PR to add a high-precision complex-valued polygamma would be highly appreciated. Looking forward to it!