try fix docs build

Author: nkrusch

.travis.yml

@@ -4,7 +4,7 @@ node_js:
   - "14.15.1"
 
 python:
-  - "3.7"
+  - "3.7.3"
 
 jobs:
   include:
@@ -15,7 +15,7 @@ jobs:
         - npm run test:travis || travis_terminate 1
 
     - language: python
-      python: 3.7
+      python: 3.7.3
       script:
         - pip install -r requirements.txt
         - mkdocs build