Add linters, formatters, pre-commit hooks and Github Actions

.flake8

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+[flake8]
+max-line-length = 100
+select = C,E,F,W,B,B950
+ignore = E203, E501, W503, E741, B008
+per-file-ignores =
+  **/__init__.py: F403,F405

.github/workflows/summon.yml

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+# vim: set ft=yaml ts=2 sw=2:
+
+name: Summon Tasks
+
+on: [push]
+
+jobs:
+  lint:
+    strategy:
+      matrix:
+        python: ['3.7', '3.10']
+        os: [ubuntu-latest]
+
+    name: Static checks
+
+    runs-on: ${{ matrix.os }}
+
+    steps:
+      - uses: actions/checkout@v2
+
+      - name: Set up Python ${{ matrix.python }}
+        uses: actions/setup-python@v1
+        with:
+          python-version: ${{ matrix.python }}
+
+      - name: Install dependencies
+        run: |
+          python -m pip install --upgrade pip
+          pip install poetry
+          poetry install
+
+      - name: Run static checks
+        run: poetry run summon static-checks
+
+
+  # test:
+  #   strategy:
+  #     matrix:
+  #       python: ['3.10']
+  #       os: [ubuntu-latest, windows-latest]
+
+  #   name: Python ${{ matrix.python }} on ${{ matrix.os }}
+
+  #   runs-on: ${{ matrix.os }}
+
+  #   steps:
+  #     - uses: actions/checkout@v2
+
+  #     - name: Set up Python ${{ matrix.python }}
+  #       uses: actions/setup-python@v1
+  #       with:
+  #         python-version: ${{ matrix.python }}
+
+  #     - name: Install dependencies
+  #       run: |
+  #         python -m pip install --upgrade pip
+  #         pip install poetry
+  #         poetry install
+
+  #     - name: Run tests
+  #       run: poetry run summon test --coverage
+
+  #     - name: Generate coverage.xml
+  #       run: poetry run coverage xml
+
+  #     - uses: codecov/codecov-action@v1
+  #       with:
+  #         fail_ci_if_error: false  # Setting this to true is a headache.

.pre-commit-config.yaml

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+repos:
+- hooks:
+  - {id: check-yaml}
+  - {id: end-of-file-fixer}
+  - {id: trailing-whitespace}
+  repo: https://github.com/pre-commit/pre-commit-hooks
+  rev: v4.2.0
+- hooks:
+  - entry: poetry run summon lint --no-full-report
+    id: linters
+    language: system
+    name: Lint
+    require_serial: true
+    types: [python]
+  - entry: poetry run summon format
+    id: formatters
+    language: system
+    name: Format
+    types: [python]
+  repo: local