Invalid Escape Sequences in Python 3.6

[cooperlees]

I'd like to fix the following invalid escape sequences scattered across the click code.

click/core.py:1253: DeprecationWarning: invalid escape sequence \s
"""
click/_compat.py:14: DeprecationWarning: invalid escape sequence [
_ansi_re = re.compile('\033[((?:\d|;)*)([a-zA-Z])')
click/decorators.py:111: DeprecationWarning: invalid escape sequence \s
""

If no one disagrees, I'll create a diff making these r''

[cooperlees]

Created #807

[segevfiner]

BTW, I think this will become a hard error in Python 3.7

[jcrotts]

Closing this because it was merged in #807