fix(tests): need to restore sys.meta_path along with sys.path

In the restore_import_state contextmanager, when the pre-test import
state is restored, sys.meta_path must be restored along with sys.path.

In some cases, setuptools appends it's finders to sys.meta_path.
Since restore_import_state might "unimport" setuptools after a test,
it can get confused and append its finders again in a second test.
This results in import_metadata.distributions() listing all
distributions twice. This results, e.g., in the cryptic 'RuntimeError:
Plugin "webpack-support" is already registered' test failure.

tests/conftest.py

@@ -74,6 +74,8 @@ def restore_import_state():
     """
     save_path = sys.path
     sys.path = save_path.copy()
+    save_meta_path = sys.meta_path
+    sys.meta_path = save_meta_path.copy()
 
     # Restoring `sys.modules` is an attempt to unload any
     # modules loaded during the test so that they can be re-loaded for
@@ -95,6 +97,7 @@ def restore_import_state():
             del sys.modules[name]
         sys.modules.update(saved_modules)
         sys.path = save_path
+        sys.meta_path = save_meta_path
 
 
 @pytest.fixture