Manage pre-commit with Poetry in GitHub Actions

https://github.com/pre-commit/mirrors-mypy/issues/3#issuecomment-547999959
pre-commit/pre-commit#880 (comment)

pre-commit runs mypy from an isolated virtualenv, so it requires a
duplicate list of Python requirements. Furthermore, these requirements
are installed by pip, which reduces compatibility with Poetry. Editable
installs require a setup.py, which is not present in Poetry projects.
The following error is seen:

```text
❯ pip install -e .

ERROR: File "setup.py" not found. Directory cannot be installed in
editable mode (A "pyproject.toml" file was found, but editable mode
currently requires a setup.py based build.)
```

There is no ideal solution to this problem. The simplest is to add
`language: system`, which means "you have to rely on whatever state the
user's machine is in" (https://github.com/pre-commit/mirrors-mypy/issues/3#issuecomment-547999959).
That's fine, and just requires an update to the GitHub Actions workflow
to install the Poetry project, as I do before running pytest.

.github/workflows/hooks.yml

@@ -9,19 +9,33 @@ on:
 jobs:
   pre-commit:
     runs-on: ubuntu-latest
+    env:
+      POETRY_VIRTUALENVS_CREATE: false
     steps:
       - uses: actions/checkout@v2
       - uses: actions/setup-python@v2
         with:
           python-version: 3.8
+      - name: Set up Poetry cache for Python dependencies
+        uses: actions/cache@v2
+        if: startsWith(runner.os, 'Linux')
+        with:
+          path: ~/.cache/pypoetry
+          key: ${{ runner.os }}-poetry-${{ hashFiles('**/poetry.lock') }}
+          restore-keys: ${{ runner.os }}-poetry-
       - name: Set up pre-commit cache
         uses: actions/cache@v2
         if: startsWith(runner.os, 'Linux')
         with:
           path: ~/.cache/pre-commit
           key: ${{ runner.os }}-pre-commit-${{ hashFiles('.pre-commit-config.yaml') }}
           restore-keys: ${{ runner.os }}-pre-commit-
+      - name: Install Poetry
+        run: |
+          curl -fsS -o get-poetry.py https://raw.githubusercontent.com/python-poetry/poetry/master/get-poetry.py
+          python get-poetry.py -y
+          echo "$HOME/.poetry/bin" >> $GITHUB_PATH
       - name: Install dependencies
-        run: python -m pip install pre-commit
+        run: poetry install --no-interaction -E fastapi
       - name: Run pre-commit hooks
         run: pre-commit run --all-files

.pre-commit-config.yaml

@@ -19,8 +19,7 @@ repos:
     rev: v0.790
     hooks:
       - id: mypy
-        additional_dependencies:
-          - pytest-mock
+        language: system
   - repo: https://github.com/prettier/prettier
     rev: 2.1.2
     hooks: