[util/pipeline] use 'Queue' without generics

Apparently Python 3.8 does not support generic type parameters on the
built-in 'queue.Queue' type?

beets/util/pipeline.py

@@ -35,7 +35,6 @@ in place of any single coroutine.
 import queue
 import sys
 from threading import Lock, Thread
-from typing import TypeVar
 
 BUBBLE = "__PIPELINE_BUBBLE__"
 POISON = "__PIPELINE_POISON__"
@@ -85,10 +84,7 @@ def _invalidate_queue(q, val=None, sync=True):
             q.mutex.release()
 
 
-T = TypeVar("T")
-
-
-class CountedQueue(queue.Queue[T]):
+class CountedQueue(queue.Queue):
     """A queue that keeps track of the number of threads that are
     still feeding into it. The queue is poisoned when all threads are
     finished with the queue.

beetsplug/replaygain.py

@@ -1181,9 +1181,7 @@ class ExceptionWatcher(Thread):
     Once an exception occurs, raise it and execute a callback.
     """
 
-    def __init__(
-        self, queue: queue.Queue[Exception], callback: Callable[[], None]
-    ):
+    def __init__(self, queue: queue.Queue, callback: Callable[[], None]):
         self._queue = queue
         self._callback = callback
         self._stopevent = Event()
@@ -1442,7 +1440,7 @@ class ReplayGainPlugin(BeetsPlugin):
         """Open a `ThreadPool` instance in `self.pool`"""
         if self.pool is None and self.backend_instance.do_parallel:
             self.pool = ThreadPool(threads)
-            self.exc_queue: queue.Queue[Exception] = queue.Queue()
+            self.exc_queue: queue.Queue = queue.Queue()
 
             signal.signal(signal.SIGINT, self._interrupt)