⬇️ Match Python version in CI to version in container (3.7.9)

> we had to skip some tests in Python 3.8 due to changes in how multiprocessing works: nipy/nipype#3154
> I'm not sure whether Nipype will work in general after Python 3.7.
― @effigies, https://mattermost.brainhack.org/brainhack/pl/kxjyek8wk3nwjmcrdyw969kfde

Author: shnizzedy

.circleci/config.yml

@@ -16,8 +16,8 @@ commands:
           name: "Setting Python Version"
           command: |
             cd /opt/circleci/.pyenv/plugins/python-build/../.. && git pull && cd -
-            pyenv install -v 3.9.0
-            pyenv global 3.9.0
+            pyenv install -v 3.7.9
+            pyenv global 3.7.9
   install-singularity-requirements:
     steps:
       - run: