ci: Ensure minimum version builds are really constrained.

QuLogic · QuLogic · commit d17cc1c5b5e5 · 2020-11-27T05:15:00.000-05:00
On Travis, NumPy was already installed and happened to be 1.16.4,
roundabout our desired minimum version. On GitHub Actions, NumPy is not
installed and gets installed later, since it was never specified in the
command with the pins.

Also, add minimum Pillow to the pins.
diff --git a/.github/workflows/tests.yml b/.github/workflows/tests.yml
@@ -100,6 +100,7 @@ jobs:
 
           # Install dependencies from PyPI.
           python -mpip install --upgrade $PRE \
+            cycler kiwisolver numpy pillow pyparsing python-dateutil \
             -r requirements/testing/travis_all.txt \
             ${{ matrix.extra-requirements }}
 
@@ -154,7 +155,9 @@ jobs:
             export CPPFLAGS=--coverage
           fi
 
-          python -mpip install -e .
+          # All dependencies must have been pre-installed, so that the minver
+          # constraints are held.
+          python -mpip install --no-deps -e .
 
           if [[ "${{ runner.os }}" != 'macOS' ]]; then
             unset CPPFLAGS
diff --git a/requirements/testing/minver.txt b/requirements/testing/minver.txt
@@ -1,7 +1,8 @@
-# Extra pip requirements for the minimum-version travis run
+# Extra pip requirements for the minimum-version CI run
 
 cycler==0.10
-python-dateutil==2.7
+kiwisolver==1.0.1
 numpy==1.16.0
+pillow==6.2.0
 pyparsing==2.2.1
-kiwisolver==1.0.1
+python-dateutil==2.7
