Add pre-commit and an initial CI

.github/workflows/ci.yml

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+name: CI
+on:
+  push:
+    branches: [master]
+  pull_request:
+    branches: [master]
+
+jobs:
+  static-analysis:
+    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"
+      - uses: pre-commit/action@v2.0.0
+        with:
+          extra_args: --all-files

.pre-commit-config.yaml

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+# See https://pre-commit.com for more information
+# See https://pre-commit.com/hooks.html for more hooks
+repos:
+  - repo: https://github.com/pre-commit/pre-commit-hooks
+    rev: v2.4.0
+    hooks:
+      - id: trailing-whitespace
+      - id: end-of-file-fixer
+      - id: check-yaml
+      - id: check-added-large-files
+  - repo: https://github.com/prettier/prettier
+    rev: 1.19.1
+    hooks:
+      - id: prettier
+  - repo: https://github.com/psf/black
+    rev: 19.3b0
+    hooks:
+      - id: black
+  - repo: https://github.com/timothycrosley/isort.git
+    rev: "4.3.21-2"
+    hooks:
+      - id: isort
+  - repo: https://github.com/pre-commit/mirrors-mypy
+    rev: v0.740
+    hooks:
+      - id: mypy
+  - repo: https://gitlab.com/pycqa/flake8.git
+    rev: 8f9b4931b9a28896fb43edccb23016a7540f5b82
+    hooks:
+      - id: flake8