Add github action to use flake8

.github/workflows/lint.yml

@@ -0,0 +1,50 @@
+name: flake8
+
+on:
+  pull_request:
+    types:
+      - 'synchronize'
+      - 'opened'
+
+jobs:
+  lint:
+
+    runs-on: ubuntu-latest
+
+    steps:
+    - uses: actions/checkout@v2
+    - name: Set up Python 3.9
+      uses: actions/setup-python@v2
+      with:
+        python-version: 3.9
+    - name: Install dependencies
+      run: |
+        python -m pip install --upgrade pip
+        pip install flake8
+    - name: Get current errors
+      # echo "CURRENT_ERROR_STR=\"$(cat $tmpafter)\"" >> $GITHUB_ENV
+      run: |
+        tmpafter=$(mktemp)
+        find src -name \*.py -exec flake8 --ignore=E402,E501,W504 {} + | egrep -v "alembic/versions/|usr/local/share/pysnmp/mibs/" > $tmpafter
+        num_errors_after=`cat $tmpafter | wc -l`
+        echo "CURRENT_ERRORS=${num_errors_after}" >> $GITHUB_ENV
+    - name: Checkout base branch
+      uses: actions/checkout@v2
+      with:
+        ref: ${{ github.base_ref }}
+    - name: Get errors from base branch
+      # echo "OLD_ERROR_STR=\"$(cat $tmpbefore)\"" >> $GITHUB_ENV
+      run: |
+        tmpbefore=$(mktemp)
+        find src -name \*.py -exec flake8 --ignore=E402,E501,W504 {} + | egrep -v "alembic/versions/|usr/local/share/pysnmp/mibs/" > $tmpbefore
+        num_errors_before=`cat $tmpbefore | wc -l`
+        echo "OLD_ERRORS=${num_errors_before}" >> $GITHUB_ENV
+    - name: Conclusion
+      # diff <(echo "${{ env.OLD_ERROR_STR }}") <(echo "${{ env.CURRENT_ERROR_str }}")
+      run: |
+        if [ ${{ env.CURRENT_ERRORS }} -gt ${{ env.OLD_ERRORS }} ]; then
+          echo "New flake8 errors were introduced"
+          exit 1
+        else
+          echo "No new flake 8 errors were introduced"
+        fi