remove abstract method annotation from copy method

The method was not abstract.

src/pluginbase.py

@@ -1,11 +1,7 @@
-from abc import abstractmethod
-
-
 class PluginBase():
     def __init__(self):
         pass
 
-    @abstractmethod
     def copy(self):
         """
         Subclasses that use state must implement this method and return a new instance of themselves.