Use python 3.11 for tests, reflecting deployment

The current production server has Python 3.11.

The change is directly motivated by a CI error (AttributeError: module
'locale' has no attribute 'getencoding') in code introduced in
f3c82f4. According to Python docs, the locale.getencoding() function
was introduced in 3.11.

Author: mslw

.github/workflows/run-server-tests.yml

@@ -9,7 +9,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        python-version: [3.7]
+        python-version: [3.11]
 
     steps:
       - uses: actions/checkout@v2