GitHub action to setup poetry (#1)

* GitHub action to setup poetry

actions/setup-python does the hard work for us, but we make use of
actions/cache to avoid having to re-install poetry and the project
unless the poetry version we're using has changed, or the hash of
the lockfile has changed.

After using this action, subsequent job steps can use `poetry run foo
[...]` to run `foo [...]` in the virtual environment.

Given that I've cited snok/install-poetry I've used theirs but stuck in
m.org as a secondary author

LICENSE

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+MIT License
+
+Copyright (c) 2020 Sondre Lillebø Gundersen
+Copyright (c) 2022 The Matrix.org Foundation C.I.C.
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+SOFTWARE.

README.md

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+This is an action which sets up Python and poetry for a project that uses [Poetry](https://python-poetry.org/) to manage its dependencies.
+It requires a `poetry.lock` file to be present in the root of the repository. The action:
+
+- calls [`actions/setup-python`](https://github.com/actions/setup-python),
+- `pip`-installs `poetry`,
+- `poetry install`-s the current project,
+- uses [`actions/cache`](https://github.com/actions/cache) to cache the poetry installation and the virtual environment it manages.
+
+From this point, the caller of the action can run commands within the poetry-managed environment with `poetry run`.
+
+The Python and Poetry versions to are configurable: see `action.yaml` for details.
+
+This particular action is loosely based on [`snok/install-poetry`](https://github.com/snok/install-poetry): in particular, the advice from its README on handling caching.

action.yml

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+name: Setup Python and Poetry
+description: Setup Python and Poetry. Cribbed from snok/install-poetry.
+inputs:
+  python-version:
+    description: Python version to pass to actions/setup-python@v2.
+    required: false
+    default: "3.x"
+  poetry-version:
+    description: Poetry version to install via pip.
+    required: false
+    default: "1.1.12"
+runs:
+  using: composite
+  steps:
+    - name: Setup Python
+      id: setup-python
+      uses: actions/setup-python@v2
+      with:
+        python-version: ${{ inputs.python-version }}
+
+    - name: Check for poetry lock
+      run: "test -f poetry.lock || (echo No lock file! && false)"
+      shell: bash
+
+    # Install poetry. It takes ~10seconds for a clean pip install, most of which is
+    # the installation rather than the downloading. So instead, we cache the user-base
+    # directory (~/.local). For this to be safe:
+    # - we must not write to `.local` as part of the CI
+    # - we must not install anything with the system `pip` other than poetry.
+    # Based on the writeup at:
+    # https://github.com/snok/install-poetry/blob/main/README.md#caching-the-poetry-installation
+    - name: Locate user site
+      id: site-user-base
+      run: echo "::set-output name=dir::$(python -m site --user-base)"
+      shell: bash
+
+    - name: Restore/cache poetry installation
+      id: poetry-install-cache
+      uses: actions/cache@v2
+      with:
+        path: ${{ steps.site-user-base.outputs.dir }}
+        key: poetry-install-cache-${{ steps.setup-python.outputs.python-version }}-${{ inputs.poetry-version }}
+
+    - name: Install poetry from scratch
+      if: "${{ steps.poetry-install-cache.outputs.cache-hit != 'true' }}"
+      run: python -m pip install --user poetry==${{ inputs.poetry-version }}
+      shell: bash
+
+    # Poetry manages a virtualenv for us. We're going to cache that too.
+    # Again, we're following snok/install-poetry's README.
+    - name: Locate poetry venv
+      id: poetry-venvs
+      run: echo "::set-output name=dir::$(python -m poetry config virtualenvs.path)"
+      shell: bash
+
+    - name: Restore/cache poetry venv
+      id: poetry-venv-cache
+      uses: actions/cache@v2
+      with:
+        path: ${{ steps.poetry-venvs.outputs.dir }}
+        key: poetry-venv-${{ runner.os }}-${{ steps.setup-python.outputs.python-version }}-${{ hashFiles('**/poetry.lock') }}
+
+    - name: Install dependencies
+      if: "${{ steps.poetry-venv-cache.outputs.cache-hit != 'true' }}"
+      run: poetry install --no-interaction --no-root
+      shell: bash
+
+    # (Not sure if this is needed if there's a cache hit?)
+    - name: Install project
+      run: poetry install --no-interaction
+      shell: bash
+
+    # For debugging---let's just check what we're working with.
+    - name: Dump virtual environment
+      run: |
+        poetry env info
+        poetry show
+      shell: bash