specify ubuntu-20.04 explicitly for python 3.6.15

ubuntu-latest apparently doesn't have that python version anymore

Author: edquist

.github/workflows/python-linters.yml

@@ -4,7 +4,7 @@ on: [push, pull_request]
 
 jobs:
   python-files:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     outputs:
       filelist: ${{ steps.python-files.outputs.filelist }}
     steps:
@@ -14,7 +14,7 @@ jobs:
         echo "::set-output name=filelist::$(find . -type f -exec awk ' /^#!.*python/{print FILENAME} {nextfile}' {} + | tr '\n' ' ')"
 
   pylint:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     needs: [python-files]
     steps:
     - uses: actions/checkout@v2
@@ -43,7 +43,7 @@ jobs:
         pylint --errors-only $PYTHON_FILES
   
   flake8:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     needs: [python-files]
     steps:
     - uses: actions/checkout@v2