bpo-29919: Remove unused imports found by pyflakes (#137)

Make also minor PEP8 coding style fixes on modified imports.
diff --git a/Lib/multiprocessing/reduction.py b/Lib/multiprocessing/reduction.py
index c043c9a..7f65947 100644
--- a/Lib/multiprocessing/reduction.py
+++ b/Lib/multiprocessing/reduction.py
@@ -7,7 +7,7 @@
 # Licensed to PSF under a Contributor Agreement.
 #
 
-from abc import ABCMeta, abstractmethod
+from abc import ABCMeta
 import copyreg
 import functools
 import io