Use venv for tests

adrienverge#232 is failing with a "This
environment is externally managed" error from pip because it's trying to
install to the system Python.

For some reason this doesn't happen on a fork, e.g.,
#2. I assume this is due to a
Github probably runner change; I'm not sure whether intentional.

Anyway we can fix this by using a venv, which is best practice anyway.

.github/workflows/tests.yaml

@@ -41,10 +41,19 @@ jobs:
         uses: actions/setup-python@v4
         with:
           python-version: ${{ matrix.pyver }}
-      - run: pip install 'pip >=23' build
-      - run: python -m build
-      - run: pip install dist/localstripe-*.tar.gz
-      - run: python -m localstripe &
+      - run: python -m venv venv
+      - run: |
+          source venv/bin/activate
+          pip install 'pip >=23' build
+      - run: |
+          source venv/bin/activate
+          python -m build
+      - run: |
+          source venv/bin/activate
+          pip install dist/localstripe-*.tar.gz
+      - run: |
+          source venv/bin/activate
+          python -m localstripe &
       # Wait for server to be up:
       - run: >
           timeout=5; while [ $((timeout--)) -ge 0 ]; do