Merge pull request #1836 from oesteban/docs/fix-1835

MAINT: pin python 3.7.4 in circle's build_docs

Author: oesteban

.circleci/config.yml

@@ -260,7 +260,7 @@ jobs:
 
   build_docs:
     docker:
-      - image: python:latest
+      - image: python:3.7.4
     working_directory: /tmp/src/fmriprep
     environment:
       - FSLOUTPUTTYPE: 'NIFTI'