Add back importlib import mode

The default import mode seems to cause the tests to fail in the CI
environment.

setup.cfg

@@ -16,7 +16,13 @@ console_scripts =
     benchcab=benchcab.main:main
 
 [tool:pytest]
-addopts = --doctest-modules --doctest-glob='*.rst' --ignore setup.py --ignore conftest.py --ignore docs/conf.py
+addopts =
+    --doctest-modules
+    --doctest-glob='*.rst'
+    --ignore setup.py
+    --ignore conftest.py
+    --ignore docs/conf.py
+    --import-mode=importlib
 
 [options]
 package_dir=