test lock to exact django version if it is a prerelease

Author: bckohan

.github/workflows/test.yml

@@ -333,7 +333,13 @@ jobs:
       - name: Install Release Dependencies
         run: |
           just setup ${{ steps.sp.outputs.python-path }}
-          just test-lock Django~=${{ matrix.django-version }}.0
+          if [[ "${{ matrix.django-version }}" =~ (a|b|rc) ]]; then
+            # prerelease, use exact version pin
+            just test-lock Django==${{ matrix.django-version }}.0
+          else
+            # stable release, use compatible release specifier
+            just test-lock Django~=${{ matrix.django-version }}.0
+          fi
       
       - name: Install mysqlclient if needed
         if: ${{ matrix.mysqlclient-version != '' }}