The code was crashing this way:

---- 8< ----
======================================================================
ERROR: test_call_ipv6 (zodburi.tests.test_resolvers.TestClientStorageURIResolver)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/kirr/src/wendelin/venv/z-dev/local/lib/python2.7/site-packages/mock/mock.py", line 1305, in patched
    return func(*args, **keywargs)
  File "/home/kirr/src/wendelin/z/zodburi/zodburi/tests/test_resolvers.py", line 265, in test_call_ipv6
    factory, dbkw = resolver('zeo://[::1]:9090?debug=true')
  File "/home/kirr/src/wendelin/z/zodburi/zodburi/resolvers.py", line 141, in __call__
    host, port = netloc.split(':')
ValueError: too many values to unpack

======================================================================
ERROR: test_call_ipv6_no_port (zodburi.tests.test_resolvers.TestClientStorageURIResolver)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/kirr/src/wendelin/venv/z-dev/local/lib/python2.7/site-packages/mock/mock.py", line 1305, in patched
    return func(*args, **keywargs)
  File "/home/kirr/src/wendelin/z/zodburi/zodburi/tests/test_resolvers.py", line 258, in test_call_ipv6_no_port
    factory, dbkw = resolver('zeo://[::1]?debug=true')
  File "/home/kirr/src/wendelin/z/zodburi/zodburi/resolvers.py", line 141, in __call__
    host, port = netloc.split(':')
ValueError: too many values to unpack
---- 8< ----

Fix it by leveraging the fact that urlparse.urlsplit return() tuple-like object
with additional .hostname and .port attributes which already parse IPv6 correctly:

In [2]: u = urlsplit('http://[::1]:5553')

In [3]: u
Out[3]: SplitResult(scheme='http', netloc='[::1]:5553', path='', query='', fragment='')

In [4]: u.netloc
Out[4]: '[::1]:5553'

In [5]: u.hostname
Out[5]: '::1'

In [6]: u.port
Out[6]: 5553

In [7]: u = urlsplit('http://[::1]')

In [8]: u.hostname
Out[8]: '::1'

In [9]: u.port

In [10]: repr(u.port)
Out[10]: 'None'