Properly close socket on destruction of OlaClient

[pietrodn]

If a program using ola.ClientWrapper is run with python -Wall on Python 3, the interpreter prints a ResourceWarning complaining about an unclosed socket.
This PR fixes that warning.

For example, if you run:

cd python
python3 -Wall examples/ola_send_dmx.py

You will get:

examples/ola_send_dmx.py:56: DeprecationWarning: tostring() is deprecated. Use tobytes() instead.
  client.SendDmx(universe, data, DmxSent)
Success!
sys:1: ResourceWarning: unclosed <socket.socket fd=5, family=AddressFamily.AF_INET, type=SocketKind.SOCK_STREAM, proto=0, laddr=('127.0.0.1', 52925), raddr=('127.0.0.1', 9010)>

With the proposed fix, the ResourceWarning disappears.

[peternewman]

Thanks @pietrodn !

[peternewman]

Would you mind retargetting this PR at our 0.10 branch please.

[pietrodn]

I cherry-picked my commit and added it on top of the 0.10 branch. Everything should be ok.