Use base.AsyncMock (for python<=37) in unit tests

Author: cderici

tests/unit/test_bundle.py

@@ -307,7 +307,7 @@ async def test_run_with_charmhub_charm(self, event_loop):
         context.trusted = False
         context.model = model
 
-        info_func = mock.AsyncMock(return_value=["12345", "name"])
+        info_func = base.AsyncMock(return_value=["12345", "name"])
 
         with patch.object(charmhub.CharmHub, 'get_charm_id', info_func):
             result = await change.run(context)
@@ -356,7 +356,7 @@ async def test_run_with_charmhub_charm_no_channel(self, event_loop):
         context.trusted = False
         context.model = model
 
-        info_func = mock.AsyncMock(return_value=["12345", "name"])
+        info_func = base.AsyncMock(return_value=["12345", "name"])
 
         with patch.object(charmhub.CharmHub, 'get_charm_id', info_func):
             result = await change.run(context)