misc: add dedicated methods for copy and deepcopy (#24033)

copy and deepcopy are recycling __getstate__ originally defined for
pickling objects; however we have a custom __getstate__ method for
formdefs because we want to save fields in a different pickle chunk.

But while removing fields in __getstate__ during pickling wouldn't
affect a "live" object, copy/deepcopy are supposed to create usable
objects, and a formdef with its fields attribute reduced to None
doesn't qualify.

Practically this could cause all sorts of malfunctions including a
not-so-subtle removal of all fields (if the copied formdef was stored
on disk).

wcs/formdef.py

@@ -1328,6 +1328,21 @@ class FormDef(StorableObject):
             if changed:
                 formdef.store()
 
+    class _EmptyClass(object):  # helper for instance creation without calling __init__
+        pass
+
+    def __copy__(self, memo=None, deepcopy=False):
+        formdef_copy = self._EmptyClass()
+        formdef_copy.__class__ = self.__class__
+        if deepcopy:
+            formdef_copy.__dict__ = copy.deepcopy(self.__dict__, memo=memo)
+        else:
+            formdef_copy.__dict__ = copy.copy(self.__dict__)
+        return formdef_copy
+
+    def __deepcopy__(self, memo=None):
+        return self.__copy__(memo=memo, deepcopy=True)
+
     # don't pickle computed attributes
     def __getstate__(self):
         odict = copy.copy(self.__dict__)