GitHub CI actions: Split tests and linters into two separate actions

We want to run our test suite across all supported Python versions, but
we don't really need/want to run our linters on more than one Python
version. Split the CI definition into:

  - a "tests" job that runs the test suite on Python versions 3.7, 3.8,
    3.9, 3.10, and 3.11, inside virtualenvs where only the test
    dependencies are available.

  - a "linter" job that runs on Python 3.10 only, inside a virtualenv
    where all dev dependencies are available.

This should hopefully make CI a little bit faster:
  - we run the linters _once_, not for every Python version
  - we run linters in parallel with tests instead of strictly afterwards
  - we run the tests in a smaller virtualenv

.github/workflows/ci.yaml

@@ -3,8 +3,7 @@ name: CI
 on: [push, pull_request]
 
 jobs:
-  ci:
-
+  tests:
     strategy:
       fail-fast: false
       matrix:
@@ -22,11 +21,33 @@ jobs:
       - uses: actions/cache@v2
         with:
           path: ~/.cache/pypoetry/virtualenvs
-          key: ${{ runner.os }}-poetry-${{ hashFiles('poetry.lock') }}
+          key: ${{ runner.os }}-poetry-tests-${{ hashFiles('poetry.lock') }}
       - name: Install project
-        run: poetry install --no-interaction --sync --with=test,dev
+        run: poetry install --no-interaction --sync --with=test
       - name: Run tests
         run: poetry run pytest
+
+  linters:
+    strategy:
+      fail-fast: false
+      matrix:
+        python-version: ["3.10"]
+        os: [ubuntu-22.04]
+    runs-on: ${{ matrix.os }}
+
+    steps:
+      - uses: actions/checkout@v3
+      - name: Set up Python ${{ matrix.python-version }}
+        uses: actions/setup-python@v4
+        with:
+          python-version: ${{ matrix.python-version }}
+      - uses: Gr1N/setup-poetry@v7
+      - uses: actions/cache@v2
+        with:
+          path: ~/.cache/pypoetry/virtualenvs
+          key: ${{ runner.os }}-poetry-linters-${{ hashFiles('poetry.lock') }}
+      - name: Install project
+        run: poetry install --no-interaction --sync --with=test,dev
       - name: Run type checker
         run: poetry run mypy
       - name: Check formatting with Black