tests: add coverage tox factor, use it on Travis

.coveragerc

@@ -1,4 +1,5 @@
 [run]
+source = parso
 
 [report]
 # Regexes for lines to exclude from consideration

.travis.yml

@@ -11,15 +11,14 @@ python:
 matrix:
   include:
     - python: 3.5
-      env: TOXENV=cov
+      env: TOXENV=py35-coverage
-  allow_failures:
-    - env: TOXENV=cov
 install:
     - pip install --quiet tox-travis
 script:
     - tox
 after_script:
-    - if [ $TOXENV == "cov" ]; then
+    - |
+      if [ "${TOXENV%-coverage}" == "$TOXENV" ]; then
         pip install --quiet coveralls;
         coveralls;
       fi

pytest.ini

@@ -1,6 +1,8 @@
 [pytest]
 addopts = --doctest-modules
 
+testpaths = parso test
+
 # Ignore broken files inblackbox test directories
 norecursedirs = .* docs scripts normalizer_issue_files build
 

tox.ini

@@ -1,19 +1,16 @@
 [tox]
-envlist = py27, py33, py34, py35, py36, py37, pypy
+envlist = {py27,py33,py34,py35,py36,py37,pypy}{,-coverage}
 [testenv]
 extras = testing
 deps =
     py26,py33: pytest>=3.0.7,<3.3
     py26,py33: setuptools<37
+    coverage: coverage
 setenv =
 # https://github.com/tomchristie/django-rest-framework/issues/1957
 # tox corrupts __pycache__, solution from here:
     PYTHONDONTWRITEBYTECODE=1
+    coverage: TOX_TESTENV_COMMAND=coverage run -m pytest
 commands =
-    pytest {posargs:parso test}
+    {env:TOX_TESTENV_COMMAND:pytest} {posargs}
-[testenv:cov]
+    coverage: coverage report
-deps =
-    coverage
-commands =
-    coverage run --source parso -m pytest
-    coverage report