[issue4126] remove not decodable environment variables

[STINNER Victor]

STINNER Victor <victor.stinner at haypocalc.com> added the comment:

See also issue #4006 which asks the opposite (keep invalid variables, 
even in os.environ(b)) :-)

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue4126>
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