fix(ci): python 3.6 no longer available

Python 3.6 x64 is no longer supported for `ubuntu-latest` (i.e.
`ubuntu-22.04`). The recommended fix is to hardcode `ubuntu-20.04`.

See: actions/setup-python#544

Author: adam-grant-hendry

.github/workflows/pythonpackage.yml

@@ -7,7 +7,7 @@ jobs:
     strategy:
       matrix:
         python-version: ["3.6", "3.7", "3.8", "3.9", "3.10"]
-        platform: [ubuntu-latest, macos-latest, windows-latest]
+        platform: [ubuntu-20.04, macos-latest, windows-latest]
     runs-on: ${{ matrix.platform }}
     steps:
     - uses: actions/checkout@v2