Use virtualenv for CI jobs

- Replaces system-wide installs with virtualenv for CI jobs.
- Improves dependency isolation and reproducibility.
- Sets up a virtualenv in each job.
- Activates the virtualenv before running commands.
- Installs dependencies within the virtualenv.

.gitea/workflows/ci.yml

@@ -7,7 +7,7 @@ on:
   pull_request:
 
 jobs:
-  setup:
+  lint:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v3
@@ -17,9 +17,15 @@ jobs:
           curl -LsSf https://astral.sh/uv/install.sh | sh
           echo "$HOME/.cargo/bin" >> $GITHUB_PATH
 
-      - name: Lint with ruff and mypy
+      - name: Set up virtualenv
         run: |
-          uv pip install --system ruff mypy
+          uv venv .venv
+          source .venv/bin/activate
+          uv pip install ruff mypy
+
+      - name: Run Ruff & Mypy
+        run: |
+          source .venv/bin/activate
           uv run ruff check
           uv run ruff format --check
           uv run mypy .
@@ -34,9 +40,15 @@ jobs:
           curl -LsSf https://astral.sh/uv/install.sh | sh
           echo "$HOME/.cargo/bin" >> $GITHUB_PATH
 
-      - name: Run tests
+      - name: Set up virtualenv
         run: |
-          uv pip install --system pytest coverage
+          uv venv .venv
+          source .venv/bin/activate
+          uv pip install pytest coverage
+
+      - name: Run Tests
+        run: |
+          source .venv/bin/activate
           uv run pytest -v --cov=. --cov-report=xml
 
       - name: Upload to Codecov