Build docs in CI on python 3.7.14

A few days ago, build docs started timing out. Analyzing the differences between the builds, it seems they started timing out when github actions switched to a new image that used python 3.7.15 instead of 3.7.14. Let's use python 3.7.14 to see if it fixes the doc build timeout.

Author: jasongrout

.github/workflows/tests.yml

@@ -11,7 +11,7 @@ jobs:
     - name: Set up Python
       uses: actions/setup-python@v4
       with:
-        python-version: 3.7
+        python-version: '3.7.14'
         cache: 'pip'
         cache-dependency-path: |
           **/setup.cfg