Upgrade the macOS VMs to use a new supported version

This change is necessary because macOS 10.15 is now deprecated.

Ref: https://github.blog/changelog/2019-10-31-github-actions-macos-virtual-environment-is-updating-to-catalina-and-dropping-mojave-supportCo-Authored-By: Hugo van Kemenade <hugovk@users.noreply.github.com>

.github/workflows/python-tests.yml

@@ -22,7 +22,7 @@ jobs:
         os:
         - ubuntu-18.04
         - ubuntu-16.04
-        - macOS-10.14
+        - macOS-latest
         # - windows-2019
         # - windows-2016
         env: