Use correct Django version in CI suite

The PEP440 syntax with a tilde ~ increments the last bit of the
version identifier. This cased all tests to run agains the latest
v2 or v3 release instead of the correct subversion and their latest
security releases.

Close #257

Author: codingjoe

.github/workflows/ci.yml

@@ -86,9 +86,9 @@ jobs:
           - "3.8"
           - "3.9"
         django-version:
-          - "2.2"
-          - "3.1"
-          - "3.2rc1"
+          - "2.2.*"
+          - "3.1.*"
+          - "3.2.*"
         extra:
           - ""
           - "progressbar"
@@ -102,7 +102,7 @@ jobs:
     - run: python -m pip install --upgrade pip setuptools codecov wheel
     - run: python -m pip install .[${{ matrix.extra }}]
       if: ${{ matrix.extra }}
-    - run: python -m pip install django~=${{ matrix.django-version }}
+    - run: python -m pip install django==${{ matrix.django-version }}
     - name: Test with pytest
       run: python setup.py test
     - run: codecov