MAINT: pin python 3.7.4 in circle's build_docs
MAINT: pin python 3.7.4 in circle's build_docs

Fixes #1835

oesteban/fmriprep,poldracklab/fmriprep,poldracklab/preprocessing-workflow,poldracklab/fmriprep,oesteban/fmriprep,poldracklab/fmriprep,poldracklab/preprocessing-workflow,oesteban/fmriprep