Fix .travis.yml and tox.ini for Python versions

- Add Python 3.6 to Travis CI
- Replace Python "nightly" by Python 3.7
  (as of today, "nightly" is still a Python 3.7.0a4+)
  and no longer allow for it to fail
- Join the pytest<3.3 version dependencies
- Fix broken dependencies:
  - pyest-timeout 1.2.1 no longer supports Python 2.6/3.4
  - tox 3.0.0 no longer suports Python 2.6/3.3
  - virtualenv 15.2 no longer supports Python 2.6/3.3
    due to wheel 0.30.0, which dropped that support

.travis.yml

@@ -7,14 +7,17 @@ python:
   - 3.3
   - 3.4
   - 3.5
-  - nightly
+  - 3.6
+  - 3.7
   - pypy
   - pypy3
 
 before_install:
   - SED_GET_PY='s/[^0-9]//g;s/.//3g;2s/.*/py/;1!G;h;$!d;s/y.2./y/;s/y.3./y3/'
   - PY=$(python -V 2>&1 | sed "$SED_GET_PY")
-  - test $PY == 32 && TOX='virtualenv<14 tox' || TOX=tox
+  - TOX=tox
+  - test $PY != 32 || TOX='virtualenv<14 tox<3'
+  - test $PY != 33 -a $PY != 26 || TOX='virtualenv<15.2 tox<3'
   - export TOXENV=py$PY
 
 install:
@@ -26,7 +29,3 @@ script:
 
 notifications:
     email: false
-
-matrix:
-    allow_failures:
-      - python: nightly

tox.ini

@@ -3,12 +3,12 @@ envlist = py{37,36,35,34,33,32,27,26,py3,py}
 
 [testenv]
 deps = pytest-cov
-       pytest-timeout
+       py{37,36,35,27,py3,py}: pytest-timeout
+       py{34,33,26}: pytest-timeout<1.2.1
        py26: unittest2
        py32: coverage<4
+       py{33,26}: pytest<3.3
        py32: pytest<3
-       py26: pytest<3.3
-       py33: pytest<3.3
 commands = py.test {posargs}
 
 [pytest]