Fix C extension import in CI tests

The C extension was being built but not imported during tests due to
Python's sys.path ordering. The project root directory was appearing
first in sys.path, causing Python to import from the source directory
(which doesn't have the compiled .so file) instead of the installed
package in site-packages.

Changes:
- Add pytest --import-mode=importlib to prevent pytest from adding the
  project root to sys.path, ensuring tests import from the installed
  package in the tox virtualenv
- Add MAXMINDDB_REQUIRE_EXTENSION=1 to the "Setup test suite" step to
  ensure the extension is built during CI setup

Author: oschwald

.github/workflows/test.yml

@@ -32,6 +32,7 @@ jobs:
       - name: Setup test suite
         run: tox run -vv --notest --skip-missing-interpreters false
         env:
+          MAXMINDDB_REQUIRE_EXTENSION: 1
           TOX_GH_MAJOR_MINOR: ${{ matrix.env }}
       - name: Run test suite
         run: tox run --skip-pkg-install

pyproject.toml

@@ -90,6 +90,13 @@ ignore = [
 "setup.py" = ["ALL"]
 "tests/*" = ["ANN201", "D"]
 
+[tool.pytest.ini_options]
+# Use importlib mode to prevent pytest from adding the project root to sys.path.
+# This ensures tests import from the tox virtualenv's installed package (which includes
+# the compiled C extension) rather than directly from the source directory (which doesn't
+# have the .so file). We still test the source code, just via the installed version.
+addopts = "--import-mode=importlib"
+
 [tool.tox]
 env_list = [
     "3.10",