CI: Actually run tests on macOS

Commit 4094cf2 moved the CI from using macos-latest to macos-10.15, but
there were a few other places in ci.yml which also needed to be changed.
Without these changes, the CI system was not actually running pytest on
macOS.

Fix this.

.github/workflows/ci.yml

@@ -50,7 +50,7 @@ jobs:
         sudo apt -y install libsdl2-mixer-2.0-0 libsdl2-image-2.0-0 libsdl2-2.0-0 libgtk2.0-0
     
     - name: Install MacOS dependencies
-      if: matrix.os == 'macos-latest'
+      if: matrix.os == 'macos-10.15'
       run: conda install python.app
 
     - name: Install dependencies
@@ -71,7 +71,7 @@ jobs:
         xvfb-run -a python -m pytest
     
     - name: Test with pytest (MacOS)
-      if: matrix.os == 'macos-latest'
+      if: matrix.os == 'macos-10.15'
       run: |
         pythonw -m pytest