Add W605 warning for invalid escape sequences in string literals

[jdufresne]

Starting with Python 3.6, invalid escape sequences in string literals are now deprecated. In a future version of Python, invalid escape sequences will be a syntax error. While this deprecation produces a
runtime warning, it only appears if warnings are enabled and the first time the Python source is compiled to byte code. By adding a check to pycodestyle, projects can take advantage of static analysis to catch and fix these future syntax errors.

For more information on the deprecation, see the Python release notes

A backslash-character pair that is not a valid escape sequence now generates a DeprecationWarning. Although this will eventually become a SyntaxError, that will not be for several Python releases.

Fixes #633

[sigmavirus24]

Why not do:

pos = string.find('\\')
while pos >= 0:
    pos += 1
    if string[pos] not in valid:
        yield (...)
    pos = string.find('\\', pos + 1)

[jdufresne]

Thanks for the review.

I agree, that solution is simpler. I've made the suggested change.