Python 3.9 not latest.

Author: jwg4

.github/workflows/minimal-test.yml

@@ -7,7 +7,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        python-version: [3.7, 3.8, latest]
+        python-version: [3.7, 3.8, 3.9]
         os: [ubuntu-18.04]
         flask-version: [latest]
     runs-on: ${{ matrix.os }}