Implement bpo-30103: Allow Uuencode in Python using backtick as zero instead of space

Merged Bitbucket Importer requested to merge branch/py3.7-bpo-30103 into branch/py3.7

Created originally on Bitbucket by Yannick_Jadoul (Yannick Jadoul)

Should fix test_uu and some of the test_binascii tests.

See https://bugs.python.org/issue30103 & https://github.com/python/cpython/pull/1326

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