usb1: Make finalizer registration thread-safe

itertools.count.__next__ is not thread-safe.
Wrapping it in a lock call seems the simplest approach.
Make counter an instance attribute rather than a class attribute, to reduce
the risk of lock contention at the expense of a slight increase in memory use.

Author: vpelletier

usb1/__init__.py

@@ -174,10 +174,10 @@ def create_binary_buffer(init_or_size):
     return create_initialised_buffer(init_or_size)
 
 class _LibUSB1Finalizer: # pylint: disable=too-few-public-methods
-    __finalizer_id_generator = itertools.count()
-
     def __init__(self):
         self._finalizer_dict = {}
+        self.__finalizer_id_generator = itertools.count()
+        self.__finalizer_id_generator_lock = threading.Lock()
 
     @staticmethod
     def __finalize(handle, pop, func, kw):
@@ -187,7 +187,8 @@ def __finalize(handle, pop, func, kw):
             pop(handle)
 
     def _getFinalizer(self, obj, func, **kw):
-        handle = next(self.__finalizer_id_generator)
+        with self.__finalizer_id_generator_lock:
+            handle = next(self.__finalizer_id_generator)
         finalizer_dict = self._finalizer_dict
         finalizer_dict[handle] = finalizer = weakref.finalize(
             obj,