ci: pin psycopg2 to 2.8

This is needed for the CI to work before
psycopg/psycopg2#1293 is fixed somehow.

.github/workflows/django.yml

@@ -38,7 +38,7 @@ jobs:
       continue-on-error: ${{ matrix.django-version == '3.1.0' || matrix.django-version == 'main' }}
       run: |
         python -m pip install --upgrade pip
-        pip install psycopg2
+        pip install 'psycopg2<2.9'
         pip install -r requirements.txt -r requirements-dev.txt
     - name: Install Django Release
       run: |