ci: Test minimum dependencies in CI (openapi-generators#1182)

This way, when we upgrade the upper limit of a dependency (and the
version we test for most of the time), we don't accidentally break our
listed minimum version.

---------

Co-authored-by: Dylan Anthony <[email protected]>

Author: dbanty

.github/workflows/checks.yml

@@ -51,15 +51,11 @@ jobs:
       - name: Run pytest without coverage
         if: matrix.os != 'ubuntu-latest'
         run: pdm test
-        env:
-          TASKIPY: true
 
       - name: Run pytest with coverage
         if: matrix.os == 'ubuntu-latest'
         run: pdm test_with_coverage
-        env:
-          TASKIPY: true
-      
+
       - run: mv .coverage .coverage.${{ matrix.python }}
         if: matrix.os == 'ubuntu-latest'
 
@@ -72,6 +68,45 @@ jobs:
           if-no-files-found: error
           include-hidden-files: true
 
+  test_min_deps:
+    strategy:
+      matrix:
+        os: [ ubuntu-latest, macos-latest, windows-latest ]
+    runs-on: ${{ matrix.os }}
+    steps:
+      - uses: actions/[email protected]
+      - name: Set up Python
+        uses: actions/[email protected]
+        with:
+          python-version: "3.9"
+
+      - name: Get Python Version
+        id: get_python_version
+        run: echo "python_version=$(python --version)" >> $GITHUB_OUTPUT
+        shell: bash
+
+      - name: Cache dependencies
+        uses: actions/cache@v4
+        with:
+          path: .venv
+          key: ${{ runner.os }}-${{ steps.get_python_version.outputs.python_version }}-min-dependencies-${{ hashFiles('**/pdm.lock') }}
+          restore-keys: |
+            ${{ runner.os }}-${{ steps.get_python_version.outputs.python_version }}-min-dependencies
+      - name: Install PDM
+        run: pip install pdm
+
+      - name: Install minimum dependencies
+        run: pdm install -L pdm.minimal.lock
+
+      - name: Run mypy
+        run: pdm mypy --show-error-codes
+
+      - name: Lint
+        run: pdm run ruff check .
+
+      - name: Run unit tests only  # snapshots are expected to fail
+        run: pdm unit_test
+
   coverage:
     name: Combine & check coverage
     needs: test