Add **kwargs to PtyProcess.spawn

[dong-zeyu]

The PtyProcessUnicode class accept keywords argument like encoding and codec_errors. However, it seems not easy to set these argument just using PtyProcess.spawn.

I think we could add a **kwargs to spawn and create the class instance using cls(pid, fd, **kwargs). It will be neat and improve extensibility.

[takluyver]

This seems reasonable to me. Sorry to have left it so long without looking at it. If you're still interested, could you add a test of using the arguments for unicode decoding?

[dong-zeyu]

Thanks for your reply. A test was added. It will try to print some non-utf8 char and check whether the error handling argument is taking effect.

[takluyver]

Hmm, it seems like there's something funny going on with the tests on MacOS 😕

[dong-zeyu]

I see. Looks like printf in MacOS does not support \xXX format to hex value. I will use octal number instead. This should be unix compatible.

[takluyver]

Thanks, this looks good to me.

There's a discussion underway in #76 about whether ptyprocess should still test on Python 2.7, so I'll hold off on merging this for now.