Add Flake8 workflow

Check our Python scripts by adding a Flake8 workflow to the GitHub
actions.

Add a config file that disables every check that currently has errors
for our Python code, so no errors should be reported initially.

The intention is to gradually re-enable these checks and fix the
corresponding issues (similarly to how we handle clang-tidy).

.flake8

@@ -0,0 +1,32 @@
+[flake8]
+exclude = .git,__pycache__,lang/json
+ignore =
+    # The following errors were present when starting to use flake8; they need
+    # to be triaged to determine which we should fix and which we should leave
+    # permanently ignored
+    E111,
+    E123,
+    E128,
+    E131,
+    E201,
+    E202,
+    E225,
+    E231,
+    E241,
+    E261,
+    E262,
+    E265,
+    E266,
+    E302,
+    E305,
+    E501,
+    E713,
+    E722,
+    E731,
+    F821,
+    F841,
+    W291,
+    W391,
+    W503,
+    W504,
+    W605

.github/workflows/flake8.yml

@@ -0,0 +1,21 @@
+name: Flake8 Python linting
+
+on:
+  push:
+    branches:
+    - master
+  pull_request:
+    branches:
+    - master
+
+jobs:
+  check:
+    runs-on: ubuntu-latest
+
+    steps:
+    - name: checkout repository
+      uses: actions/checkout@v1
+      with:
+        fetch-depth: 10
+    - name: run Flake8
+      run: flake8