Update pythonpackage.yml

Add Python versions 3.6 and 3.8.
Configure flake8 max line length to 88 as per Black formatter setting.

.github/workflows/pythonpackage.yml

@@ -1,6 +1,6 @@
 name: Python package
 
-on: [push]
+on: [push, pull_request]
 
 jobs:
   build:
@@ -9,7 +9,7 @@ jobs:
     strategy:
       max-parallel: 4
       matrix:
-        python-version: [3.7]
+        python-version: [3.6, 3.7, 3.8]
 
     steps:
     - uses: actions/checkout@v1
@@ -31,8 +31,7 @@ jobs:
         pip install flake8
         # stop the build if there are Python syntax errors or undefined names
         flake8 . --count --select=E9,F63,F7,F82 --show-source --statistics
-        # exit-zero treats all errors as warnings. The GitHub editor is 127 chars wide
-        flake8 . --count --exit-zero --max-complexity=10 --max-line-length=127 --statistics
+        flake8 . --count --exit-zero --max-complexity=10 --max-line-length=88 --statistics
     - name: Test with pytest
       env:
         COVERALLS_REPO_TOKEN: ${{ secrets.github_token }}