tag ubuntu version for 3.6

Author: TomDonoghue

.github/workflows/build.yml

@@ -11,13 +11,15 @@ on:
 jobs:
   build:
 
-    runs-on: ubuntu-latest
+    # Tag ubuntu version to 20.04, in order to support python 3.6
+    #   See issue: https://github.com/actions/setup-python/issues/544
+    #   When ready to drop 3.6, can revert from 'ubuntu-20.04' -> 'ubuntu-latest'
+    runs-on: ubuntu-20.04
     env:
       MODULE_NAME: fooof
     strategy:
       matrix:
-        python-version: [3.7, 3.8, 3.9, "3.10", "3.11"]
-        #python-version: [3.6, 3.7, 3.8, 3.9, "3.10"]
+        python-version: ["3.6", "3.7", "3.8", "3.9", "3.10", "3.11"]
 
     steps:
     - uses: actions/checkout@v3