Only patch npe1 plugin_manager when it exists (#207)

* Only patch plugin manager when it exists

* Check against version

---------

Co-authored-by: Draga Doncila <ddon0001@student.monash.edu>

Author: DragaDoncila

src/napari_plugin_manager/_tests/test_qt_plugin_dialog.py

@@ -4,8 +4,10 @@
 from collections.abc import Generator
 from unittest.mock import MagicMock, call, patch
 
+import napari
 import napari.plugins
 import npe2
+import packaging.version
 import pytest
 from napari.plugins._tests.test_npe2 import mock_pm  # noqa
 from napari.utils.translations import trans
@@ -169,8 +171,15 @@ def set_blocked(self, plugin, blocked):
     monkeypatch.setattr(
         qt_plugin_dialog, 'running_as_constructor_app', lambda: request.param
     )
+
+    napari_leq_066 = packaging.version.parse(
+        napari.__version__
+    ) <= packaging.version.parse('0.6.6')
     monkeypatch.setattr(
-        napari.plugins, 'plugin_manager', OldPluginManagerMock()
+        napari.plugins,
+        'plugin_manager',
+        OldPluginManagerMock(),
+        raising=napari_leq_066,
     )
 
     monkeypatch.setattr(importlib.metadata, 'metadata', mock_metadata)