Don't use gammaln on complex input

Don't use gammaln on complex input

In scipy >= 1.0, it rejects such input with
TypeError: ufunc 'gammaln' not supported for the input types, and the
inputs could not be safely coerced to any supported types according
to the casting rule ''safe''

Author: Kevin "bashtage" Sheppard, Rebecca N. Palmer <rebecca_palmer@zoho.com>
Origin: upstream https://github.com/statsmodels/statsmodels/pull/3964/commits/c11c6242ff81a0f83d3e7fd83a8105397d3cbc1d
Forwarded: not-needed

Gbp-Pq: Name gammaln_complex.patch