[issue28128] Improve the warning message for invalid escape sequences

[Serhiy Storchaka]

Changes by Serhiy Storchaka <storchaka+cpython at gmail.com>:


Added file: http://bugs.python.org/file45287/28128-3.diff

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue28128>
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