Add: [Actions] CodeQL, flake8, black, and regression tests

.flake8

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+[flake8]
+max-line-length = 120
+inline-quotes = double
+
+# We use 'black' for coding style; these next two warnings are not PEP-8
+# compliant.
+ignore = E203, W503

.github/workflows/commit-checker.yml

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+name: Commit checker
+
+on:
+  pull_request:
+
+jobs:
+  commit-checker:
+    name: Commit checker
+    runs-on: ubuntu-latest
+
+    steps:
+    - name: Checkout
+      uses: actions/checkout@v2
+      with:
+        fetch-depth: 4
+
+    - name: Get pull-request commits
+      run: |
+        # actions/checkout did a merge checkout of the pull-request. As such, the first
+        # commit is the merge commit. This means that on HEAD^ is the base branch, and
+        # on HEAD^2 are the commits from the pull-request. We now check if those trees
+        # have a common parent. If not, we fetch a few more commits till we do. In result,
+        # the log between HEAD^ and HEAD^2 will be the commits in the pull-request.
+        DEPTH=4
+        while [ -z "$(git merge-base HEAD^ HEAD^2)" ]; do
+            git -c protocol.version=2 fetch --no-tags --prune --progress --no-recurse-submodules --deepen=${DEPTH} origin HEAD
+            DEPTH=$(( ${DEPTH} * 4 ))
+        done
+
+        # Just to show which commits we are going to evaluate.
+        echo "Commits in this pull-request:"
+        git log --oneline HEAD^..HEAD^2
+
+    - name: Checkout commit-checker
+      uses: actions/checkout@v2
+      with:
+        repository: OpenTTD/OpenTTD-git-hooks
+        path: git-hooks
+        ref: master
+
+    - name: Check commits
+      run: |
+        HOOKS_DIR=./git-hooks/hooks GIT_DIR=.git ./git-hooks/hooks/check-commits.sh HEAD^..HEAD^2
+        echo "Commit checks passed"

.github/workflows/testing.yml

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+name: Testing
+
+on:
+  push:
+    branches:
+    - master
+  pull_request:
+
+jobs:
+  codeql:
+    name: Security and Quality
+    runs-on: ubuntu-latest
+    steps:
+    - name: Checkout
+      uses: actions/checkout@v2
+      with:
+        # Must fetch at least the immediate parents so that if this is
+        # a pull request then we can checkout the head of the pull request.
+        fetch-depth: 2
+    # If this run was triggered by a pull request event then checkout
+    # the head of the pull request instead of the merge commit.
+    - run: git checkout HEAD^2
+      if: ${{ github.event_name == 'pull_request' }}
+    - name: Set up Python 3.7
+      uses: actions/setup-python@v1
+      with:
+        python-version: 3.7
+    - name: Install dependencies
+      run: python -m pip install -e .
+    - name: Initialize CodeQL
+      uses: github/codeql-action/init@v1
+      with:
+        languages: python
+        queries: security-and-quality
+    - name: Perform CodeQL Analysis
+      uses: github/codeql-action/analyze@v1
+
+  flake8:
+    name: Flake8
+    runs-on: ubuntu-latest
+    steps:
+    - name: Checkout
+      uses: actions/checkout@v2
+    - name: Flake8
+      uses: TrueBrain/actions-flake8@master
+      with:
+        path: examples openttd_helpers regression_runner
+
+  black:
+    name: Black
+    runs-on: ubuntu-latest
+    steps:
+    - name: Checkout
+      uses: actions/checkout@v2
+    - name: Set up Python 3.7
+      uses: actions/setup-python@v1
+      with:
+        python-version: 3.7
+    - name: Black
+      run: |
+        python -m pip install --upgrade pip
+        pip install black
+        black -l 120 --check examples openttd_helpers regression_runner
+
+  regression:
+    name: Regression
+    runs-on: ubuntu-latest
+    steps:
+    - name: Checkout
+      uses: actions/checkout@v2
+    - name: Set up Python 3.7
+      uses: actions/setup-python@v1
+      with:
+        python-version: 3.7
+    - name: Set up packages
+      run: |
+        python -m pip install --upgrade pip
+        pip install -e .
+        pip install -r regression_runner/requirements.txt
+    - name: Regression
+      run: |
+        make regression