Refactor tox and ci scripts

Make sure a plain "tox" command works in all platforms without failures,
which facilitates making releases using a "devpi upload/test" workflow

* Fix small flakes failures
* Limit py35 testing to pytest>=2.7, as pytest<2.7 does not work on py35
* pexpect environments only work on Linux platforms and should be skipped on Windows
* Simplify AppVeyor script by running all tox environments: no need to have a build matrix
  because AppVeyor does not execute builds in parallel
* Add all environments to travis.yml, obtained from "tox --listenvs"

testing/test_remote.py

@@ -2,9 +2,10 @@ import py
 from xdist.slavemanage import SlaveController, unserialize_report
 from xdist.remote import serialize_report
 import execnet
-queue = py.builtin._tryimport("queue", "Queue")
 import marshal
 
+queue = py.builtin._tryimport("queue", "Queue")
+
 WAIT_TIMEOUT = 10.0