remove an invalid escape sequence

[wimglenn]

code in current release is raising warnings:

python3.6/site-packages/anytree/resolver.py:222: DeprecationWarning: invalid escape sequence \Z

I've converted this sequence to a raw string.

[coveralls]

Coverage remained the same at 100.0% when pulling d6c8691 on wimglenn:patch-1 into 9e36d88 on c0fec0de:master.

[c0fec0de]

Thanks. Fix will be part of next release in the next days...