Use a default_factory for dict default values

`backlash_steps` and `axis_inverted` were `lt.setting`s with default values that were dictionaries. This means they had mutable default values, which caused crosstalk in the test suite.

This commit swaps them to default factory functions. This should isolate instances from each other and stop the offending behaviour.

src/openflexure_microscope_server/things/stage/__init__.py

@@ -147,7 +147,7 @@ class BaseStage(lt.Thing):
         return self._apply_axis_direction(self._hardware_position)
 
     backlash_steps: dict[str, int] = lt.setting(
-        default={"x": 200, "y": 200, "z": 200}, readonly=True
+        default_factory=lambda: {"x": 200, "y": 200, "z": 200}, readonly=True
     )
     """The number of steps to elimate backlash. The sign sets the direction.
 
@@ -161,7 +161,7 @@ class BaseStage(lt.Thing):
     """Whether the stage is in motion."""
 
     axis_inverted: dict[str, bool] = lt.setting(
-        default={"x": False, "y": False, "z": False}, readonly=True
+        default_factory=lambda: {"x": False, "y": False, "z": False}, readonly=True
     )
     """Used to convert coordinates between the program frame and the hardware frame."""
 

src/openflexure_microscope_server/things/stage/dummy.py

@@ -77,7 +77,7 @@ class DummyStage(BaseStage):
             self._move_thread.join()
 
     axis_inverted: dict[str, bool] = lt.setting(
-        default={"x": True, "y": False, "z": False}, readonly=True
+        default_factory=lambda: {"x": True, "y": False, "z": False}, readonly=True
     )
     """Used to convert coordinates between the program frame and the hardware frame."""
 

src/openflexure_microscope_server/things/stage/sangaboard.py

@@ -76,7 +76,7 @@ class SangaboardThing(BaseStage):
             self._sangaboard.close()
 
     axis_inverted: dict[str, bool] = lt.setting(
-        default={"x": True, "y": False, "z": True}, readonly=True
+        default_factory=lambda: {"x": True, "y": False, "z": True}, readonly=True
     )
     """Used to convert coordinates between the program frame and the hardware frame."""