fix syntax

Author: adamchainz

.github/workflows/python-app.yml

@@ -78,7 +78,7 @@ jobs:
         sudo apt-get install -y libproj-dev libgeos-dev gdal-bin libgdal-dev
         python -m pip install --upgrade pip
         pip install -U flake8 psycopg2-binary argparse
-        pip install -U Django~=${{ matrix.django_version }}.0
+        pip install -U Django~=${{ matrix.django_version }}.0a1
     - name: Lint with flake8
       run: |
         flake8 --ignore=E501,W504 leaflet