Removing txipc in python

["Michael A. Smith" <mi...@smith-li.com>]

Both the python implementations of avro include a txipc module that
seems intended to produce an asynchronous IPC transceiver. But I'm
doubtful that either of them are functional.

1. They both require twisted and zope.interface, which are
undocumented and unmanaged by the packages' setup code.
2. There aren't any tests that cover them.
3. The python3 txipc implementation of zope.interface is obsolete, so
you can't import the module.

For my two cents, these modules are a very low maintenance priority,
and I'm inclined to remove them
(https://github.com/apache/avro/pull/716).

That said, I know people are often interested in specific skill-honing
opportunities, and this is a really good opportunity for someone to
use Python3's asyncio module to make a more modern implementation.

If you have a particular interest in maintaining (or writing anew) an
asynchronous avro IPC transceiver in Python 3, please reply so we can
work out how to proceed.

Thanks,
Michael A. Smith