ci: use CodeQL instead of LGTM

As LGTM is going to be shut down by EOY[0], let's move the code scanning to
CodeQL as recommended. Thanks to GH integration the results from such
scans will be shown both in the respective PR and in the Security ->
Code Scanning tab[1].

[0] https://github.blog/2022-08-15-the-next-step-for-lgtm-com-github-code-scanning/
[1] https://github.com/systemd/mkosi/security/code-scanning

.github/workflows/codeql.yml

@@ -0,0 +1,47 @@
+---
+# vi: ts=2 sw=2 et:
+#
+name: "CodeQL"
+
+on:
+  push:
+    branches:
+      - main
+  pull_request:
+    branches:
+      - main
+
+permissions:
+  contents: read
+
+jobs:
+  analyze:
+    name: Analyze
+    runs-on: ubuntu-22.04
+    concurrency:
+      group: ${{ github.workflow }}-${{ matrix.language }}-${{ github.ref }}
+      cancel-in-progress: true
+    permissions:
+      actions: read
+      security-events: write
+
+    strategy:
+      fail-fast: false
+      matrix:
+        language: ['python']
+
+    steps:
+    - name: Checkout repository
+      uses: actions/checkout@v3
+
+    - name: Initialize CodeQL
+      uses: github/codeql-action/init@v2
+      with:
+        languages: ${{ matrix.language }}
+        queries: +security-extended,security-and-quality
+
+    - name: Autobuild
+      uses: github/codeql-action/autobuild@v2
+
+    - name: Perform CodeQL Analysis
+      uses: github/codeql-action/analyze@v2