[2.7] bpo-31271: Fix an assertion failure in io.TextIOWrapper.write. (GH-3201)

[miss-islington]

(cherry picked from commit a5b4ea1)

https://bugs.python.org/issue31271

[the-knights-who-say-ni]

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[vstinner]

Test failed:

======================================================================

FAIL: test_illegal_encoder (test.test_io.CTextIOWrapperTest)

----------------------------------------------------------------------

Traceback (most recent call last):

  File "/home/travis/build/python/cpython/Lib/test/test_io.py", line 2675, in test_illegal_encoder

    self.assertRaises(TypeError, t.write, 'bar')

AssertionError: TypeError not raised

======================================================================

FAIL: test_illegal_encoder (test.test_io.PyTextIOWrapperTest)

----------------------------------------------------------------------

Traceback (most recent call last):

  File "/home/travis/build/python/cpython/Lib/test/test_io.py", line 2675, in test_illegal_encoder

    self.assertRaises(TypeError, t.write, 'bar')

AssertionError: TypeError not raised

[Mariatta]

Thanks @Haypo. Does @orenmn (who made the PR) or @serhiy-storchaka (who merged the PR) want to prepare the backport PR?

I also opened python/miss-islington#26 for some ideas of what @miss-islington can do in the future to handle this situation.