Determine location of module by looking at module attributes

Instead of version checking. Work around the circularity of importing
new labscript_devices (which imports labscript_utils.device_registry) by
unconditionally importing the new module first, rolling it back if it
turns out labscript_devices is old and defines the device registry
itself.

Author: chrisjbillington

labscript_utils/device_registry/__init__.py

@@ -1,21 +1,15 @@
-try:
-    import importlib.metadata as importlib_metadata
-except ImportError:
-    import importlib_metadata
-import packaging.version
+from ._device_registry import *
 
-use_local_machinery = True
+# Backwards compatibility for labscript-devices < 3.1. If labscript_devices defines the
+# device registry as well, undo the above import and use the contents of
+# labscript_devices instead. The above import must be done first so that the names are
+# available to labscript_devices during the below import, since as of 3.1 it imports
+# this module as well.
 try:
-    labscript_device_version_str = importlib_metadata.version('labscript_devices')
-    labscript_devices_version = packaging.version.parse(labscript_device_version_str)
-    use_local_machinery = labscript_devices_version >= packaging.version.parse('3.1.0.dev16')
-except importlib_metadata.PackageNotFoundError:
+    from labscript_devices import ClassRegister
+    if ClassRegister.__module__ == 'labscript_devices':
+        for name in _device_registry.__all__:
+            del globals()[name]
+        from labscript_devices import *
+except ImportError:
     pass
-
-# import and use the labscript_devices code if an old version of the package is used.
-# This ensures that new labscript_utils with old labscript_devices doesn't attempt to
-# register labscript_devices classes twice
-if not use_local_machinery:
-    from labscript_devices import * 
-else:
-    from ._device_registry import *

labscript_utils/device_registry/_device_registry.py

@@ -1,5 +1,4 @@
 import os
-import sys
 import importlib
 import imp
 import warnings
@@ -45,6 +44,20 @@ class belong to each device. Tab and parser classes must be passed to register_c
 """
 
 
+__all__ = [
+    'LABSCRIPT_DEVICES_DIRS',
+    'labscript_device',
+    'BLACS_worker',
+    'BLACS_tab',
+    'runviewer_parser',
+    'import_class_by_fullname',
+    'deprecated_import_alias',
+    'get_BLACS_tab',
+    'get_runviewer_parser',
+    'register_classes',
+]
+
+
 def _get_import_paths(import_names):
     """For the given list of packages, return all folders containing their submodules.
     If the packages do not exist, ignore them."""