Closes 0rpc#228

Travis changed the default distro to Xenial which doesn't have python
2.6 environment.

To see the installed package versions

Fix simple typo: sream -> stream

Using trusty as distro for python 2.6 build

Kill a depreciation warning by importing "collections.abc" and
fallback to "collections" on failure in order to maintain support
for older versions of Python.

Fixes: 0rpc#231
Signed-off-by: Cedric Hombourger <[email protected]>

try collections.abc instead of collections for Python >= 3.4

Python 3.7 made "async" a reserved keyword: this causes a syntax
error when passing "async" as a named parameter in remote procedure
calls. Make ClientBase.__call__() understand both "async" and
"async_" so we don't break our binary interface with older clients.

Fixes: 0rpc#239
Signed-off-by: Cedric Hombourger <[email protected]>

core: handle both "async" and "asynchronous" in remote calls

Signed-off-by: Tomasz Kłoczko <[email protected]>

Move from deprecated nose to pytest (solves 0rpc#243)

With pyzmq 23.0.0, constants were changed to enums and moved to the
constants module. Attempt to import all globals from it into our zmq
wrapper.

Closes: 0rpc#251
Signed-off-by: Cedric Hombourger <[email protected]>

There are small typos in:
- doc/protocol.md
- zerorpc/core.py

Fixes:
- Should read `transferred` rather than `transfered`.
- Should read `occurred` rather than `occured`.

Signed-off-by: Tim Gates <[email protected]>

docs: Fix a few typos

gevent_zmq: import enums from pyzmq >= 23.0.0