Improve tox ini (#2031)

* Improve tox.ini to be able to run only some tests

This change will make tox use posargs if there are any or fallback to
"tests" args (which will run the all the tests).

You can now use this command to start a specific test:

tox -e py35 -- -x tests/test_client_connection.py

* Add Python 3.6 to the testing environment

tox.ini

@@ -1,6 +1,6 @@
 [tox]
 
-envlist = check, {py34,py35}-{debug,release}-{cchardet,cython,pure}, report
+envlist = check, {py34,py35,py36}-{debug,release}-{cchardet,cython,pure}, report
 
 [testenv]
 
@@ -17,7 +17,7 @@ deps =
 commands =
     # --cov={envsitepackagesdir}/tests
     # py.test --cov={envsitepackagesdir}/aiohttp tests {posargs}
-    coverage run -m pytest tests {posargs}
+    coverage run -m pytest {posargs:tests}
     mv .coverage .coverage.{envname}
 
 setenv =
@@ -27,6 +27,7 @@ setenv =
 basepython:
     py34: python3.4
     py35: python3.5
+    py36: python3.6
 
 whitelist_externals =
     coverage
@@ -47,7 +48,7 @@ commands =
     coverage erase
 
 basepython:
-    python3.5
+    python3.6
 
 [testenv:report]
 
@@ -58,4 +59,4 @@ commands =
     echo "open file://{toxinidir}/coverage/index.html"
 
 basepython:
-    python3.5
+    python3.6