Elaborate on how Python 3.6 and 3.7 are tested in GitHub Actions (#172)

Author: vstinner

.github/workflows/build.yml

@@ -21,6 +21,9 @@ jobs:
         python:
           # Python versions (CPython):
           # https://raw.githubusercontent.com/actions/python-versions/main/versions-manifest.json
+          #
+          # Python 3.7 is tested on Ubuntu 22.04 (below).
+          # Python 3.6 is tested on Windows (below).
           - "3.8"
           - "3.9"
           - "3.10"
@@ -97,6 +100,7 @@ jobs:
 
           # Ubuntu: test deadsnakes Python versions which are not supported by
           # GHA python-versions.
+          # https://launchpad.net/~deadsnakes/+archive/ubuntu/ppa
           - os: ubuntu-22.04
             python: "3.7"