[github-actions] Use ubuntu-20.04 for Python 3.6 and 3.7

.github/workflows/python-package-ci.yml

@@ -11,11 +11,19 @@ on:
 
 jobs:
   build:
-    runs-on: ubuntu-latest
+    # The latest ubuntu release may exclude Python versions that have reached
+    # the end of life. An example of this is the upgrade from ubuntu-20.04 to
+    # ubuntu-22.04: support for Python 3.6.* and 3.7.* has been dropped and
+    # this broke our Github actions. We should periodically revisit this to
+    # make sure PyMTL works on the latest ubuntu LTS release. As of now I'm
+    # targeting ubuntu-20.04 because lots of PyMTL users still work with
+    # Python 3.6 and 3.7.
+    # runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     strategy:
       fail-fast: false
       matrix:
-        python-version: [ 3.6.15, 3.7.17, 3.8, 3.9 ] # FIXME github recognizes 3.10 as 3.1 lol
+        python-version: [ 3.6, 3.7, 3.8, 3.9 ] # FIXME github recognizes 3.10 as 3.1 lol
 
     steps:
     - uses: actions/checkout@v2