Bug in `update_ip` for Python 3

[asonnino]

In function update_ip of client.py, the following check does not work for python 3:

if (
    subdomains is None or
    (isinstance(subdomains, (unicode, str)) and r_name == subdomains) or
     r_name in subdomains
):

This seems a bug as the rest of the function is careful in specifying behaviours for both python 2 and python 3. That is easy to fix, the check could simply become as follows for python 3:

if (
    subdomains is None or
    (isinstance(subdomains, str) and r_name == subdomains) or
     r_name in subdomains
):