Add flake8 in CI

.flake8

@@ -1,2 +1,8 @@
 [flake8]
-max-line-length = 100
+ignore = C901, W504
+max-line-length = 125
+exclude = .git,__pycache__,old,build,dist,*/migrations/*.py
+max-complexity = 10
+per-file-ignores =
+    /**/tests/*_mock_data.py: E501
+    **/snap_test_*.py: E501

.github/pull_request_template.md

@@ -0,0 +1,25 @@
+Addresses xxxxxx (eg: #1 the-deep-nlp/core-server#1) \
+Depends on xxxxxx (eg: #1 the-deep-nlp/core-server/#1)
+
+## Changes
+
+* Detailed list or prose of changes
+* Breaking changes
+* Changes to configurations
+
+Mention related users here if any.
+
+## This PR doesn't introduce any:
+
+- [ ] temporary files, auto-generated files or secret keys
+- [ ] n+1 queries
+- [ ] flake8 issues
+- [ ] `print`
+- [ ] typos
+- [ ] unwanted comments
+
+## This PR contains valid:
+
+- [ ] tests
+- [ ] permission checks (tests here too)
+- [ ] translations

.github/workflows/flake8.yml

@@ -0,0 +1,25 @@
+name: Flake8 check.
+
+on:
+  pull_request:
+  push:
+    branches:
+      - main
+
+jobs:
+  check:
+    name: Flake8 check
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+
+      # Flake8 check
+      - name: Install flake8
+        run: pip install flake8
+      - name: Run flake8
+        uses: suo/flake8-github-action@releases/v1
+        with:
+          # NOTE: this needs to be the same as the job name
+          checkName: 'Flake8 check'
+        env:
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}