…. Modified the print statement too.

…op test added. Clients communication test added.

…r incoming registrations in case of missing Tkinter.

…the doctest errors.

…nt with Python 3

… servers).

… of __init__, so the the garbage collector doesn't fails if the hub initialization raises an exception.

…ening.

…pen a socket.

This was discussed and agreed on in GH astropy#972.
In addition to the structure suggested there by @lpaioro,
I put the constants and errors in separate files.

cd astropy/vo/samp
autopep8 -i --ignore E501,E126,E128,E225,E226,E231 -r . tests/

…methods with the same signature