time_test fails when running after any test which uses strptime().

The bug can be easily reproduced by running test_time twice:

$ TZ=Europe/Kiev ./python -m test.regrtest -ugui -v test_time test_time
...
======================================================================
FAIL: test_strptime (test.test_time.TimeTestCase)
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Traceback (most recent call last):
  File "/home/serhiy/py/cpython-2.7/Lib/test/test_time.py", line 120, in test_strptime
    (format, strf_output))
AssertionError: conversion specifier '%Z' failed with 'MSK' input.

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All works on 3.x and with TZ=UTC.

May be this is related to issue13309.