TST: Use Ubuntu 20.04 for running CI test suite (#1452)

Closes #1451

This sets the test suite to use ubuntu-20.04 as opposed to ubuntu-latest as the latter switched over in the last day or so to using ubuntu-22.04 and that broke testing Python 3.6. Ubuntu 20.04 has maintenance fixes for another ~2.5 years, and is supported critically for another 5 years after that

Signed-off-by: Matthew Peveler <[email protected]>

Signed-off-by: Matthew Peveler <[email protected]>

Author: MasterOdin

.github/workflows/github-ci.yaml

@@ -18,7 +18,7 @@ on:
 jobs:
   tests:
     name: pytest on ${{ matrix.python-version }}
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     strategy:
       matrix:
         python-version: ["3.6", "3.7", "3.8", "3.9", "3.10", "3.11"]