bpo-46787: Fix ProcessPoolExecutor exception memory leak (GH-31408) (GH-31408)

Do not store `ProcessPoolExecutor` work item exception traceback that prevents
exception frame locals from being garbage collected.
(cherry picked from commit 9c204b148f)

Co-authored-by: themylogin <themylogin@gmail.com>

Lib/concurrent/futures/process.py

@@ -126,6 +126,9 @@ def __init__(self, exc, tb):
         tb = traceback.format_exception(type(exc), exc, tb)
         tb = ''.join(tb)
         self.exc = exc
+        # Traceback object needs to be garbage-collected as its frames
+        # contain references to all the objects in the exception scope
+        self.exc.__traceback__ = None
         self.tb = '\n"""\n%s"""' % tb
     def __reduce__(self):
         return _rebuild_exc, (self.exc, self.tb)