Fix python version to 3.7.7

PyYAML causes issues only for tests running on python 3.7.
A suggested change to fix this issue is fixing the python 3.7
version to the 3.7.7 minor release since test start to fail
only for 3.7.8

see: actions/runner-images#1202

.github/workflows/continuous_integration.yml

@@ -33,7 +33,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        python-version: [3.6, 3.7, 3.8]
+        python-version: [3.6, 3.7.7, 3.8]
     services:
       postgres:
         image: postgres:10