BLD: investigate / switch to using miniconda rather than whls

we have a nice build system, though switching versions of sub-libraries requires rebuilding a whl (not that big a deal really).

that said, this is somewhat simpler as conda is pulling something like a whl which they are mainting, so switching versions of sub-libraries would be pretty easy.

https://github.com/statsmodels/statsmodels/commit/36bd0c9f8f2431d461d779c09f97f31d49033c97
