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"

tox.ini

@@ -1,20 +1,27 @@
 [tox]
+# if you change the envlist, please update .travis.yml file as well
 envlist=
-  py{26,27,34,35}-pytest2{4,5,6,7,8},py{27,34}-pytest28-pexpect,flakes,readme
+  py{26,27,34}-pytest2{4,5,6,7,8}
+  py35-pytest2{7,8}
+  py{27,35}-pytest28-pexpect
+  flakes
+  readme  
 
 
 [testenv]
 changedir=testing
 passenv = USER USERNAME
 deps =
-    pycmd
-    setuptools_scm # to avoid .eggs
-    pytest24: pytest~=2.4.0
-    pytest25: pytest~=2.5.0
-    pytest26: pytest~=2.6.1
-    pytest27: pytest~=2.7.2
-    pytest28: pytest~=2.8.3
-    pexpect: pexpect
+  pycmd
+  setuptools_scm # to avoid .eggs
+  pytest24: pytest~=2.4.0
+  pytest25: pytest~=2.5.0
+  pytest26: pytest~=2.6.1
+  pytest27: pytest~=2.7.2
+  pytest28: pytest~=2.8.3
+  pexpect: pexpect
+platform=
+  pexpect: linux|darwin    
 commands=
   # always clean to avoid code unmarshal mismatch on old python/pytest
   py.cleanup -aq