[issue27030] Remove deprecated re features

[Serhiy Storchaka]

Serhiy Storchaka added the comment:

If you insist I could revert converting warnings to errors (only in replacement string or all?) in 3.6. But I think they should left errors in 3.7. The earlier we make undefined escapes the errors, the earlier we can define new special escape sequences without confusing users. It is bad if the escape sequence is valid in two Python versions but has different meaning.

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<http://bugs.python.org/issue27030>
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