The way the string returned by __str__ is created fails with
Python 3.

Py3 compatibility

See merge request !10

If no context given, use the global instance.

This avoids creating multiple context when not needed and gives
the class user the responsibility to handle multiple context if
needed.

This also fixes the deadlock where at some point one socket object
was not yet deleted at the time of the context deletion thus
making the zmq_ctx_destroy method call hang forever.

The string types being different the hash will not generate
the same thing between the two versions of python.

From python 2 to 3, items became a view.