TypeError: unsupported operand type(s) for -=: 'Retry' and 'int'

The bug is impacting RoboBrowser 0.5.3 used with Python 3.4.1

robobrowser/browser.py 's __init__ is setting "self.session.adapters[protocol].max_retries" to an instance of requests.packages.urllib3.util.retry.Retry

However, Python3's urlopen() in urllib3/connectionpool.py is doing a comparison "if not isinstance(retries, Retry)" where Retry is actually 'urllib3.util.retry.Retry', leading to the exception later on.

A temporary workaround for RoboBrowser users:
```
from urllib3.util.retry import Retry
br = RoboBrowser(parser='html.parser', tries=9999, timeout=25)

retry = Retry(9999, backoff_factor=1)
for protocol in ['http://', 'https://']:
br.session.adapters[protocol].max_retries = retry
```