[3.6] bpo-31583: Fix 2to3 for using with --add-suffix option (GH-3758)

[miss-islington]

(cherry picked from commit e3a523a)

Co-authored-by: Denis Osipov osipov_d@list.ru

https://bugs.python.org/issue31583

[ambv]

Hm. The failure on Travis is unrelated:

======================================================================
ERROR: test_ignore (test.test_multiprocessing_forkserver.TestIgnoreEINTR)
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Traceback (most recent call last):
  File "/home/travis/build/python/cpython/Lib/test/_test_multiprocessing.py", line 4050, in test_ignore
    os.kill(p.pid, signal.SIGUSR1)
ProcessLookupError: [Errno 3] No such process

[miss-islington]

@denis-osipov and @ambv: Backport status check is done, and it's a failure ❌ .

[miss-islington]

@denis-osipov and @ambv: Backport status check is done, and it's a success ✅ .

[miss-islington]

@denis-osipov and @ambv: Backport status check is done, and it's a success ✅ .

[miss-islington]

Thanks, @ambv!