make testing easier

We always know that our test are under `test/`.
This allows us to call individual test files:
  tox -e py37 -- test/test_closed_streams.py

Author: Kriechi

setup.cfg

@@ -1,2 +1,5 @@
+[tool:pytest]
+testpaths = test
+
 [wheel]
 universal = 1

tox.ini

@@ -4,12 +4,12 @@ envlist = py27, py34, py35, py36, py37, pypy, lint, packaging, docs
 [testenv]
 deps= -r{toxinidir}/test_requirements.txt
 commands=
-    coverage run -m py.test {posargs} {toxinidir}/test/
+    coverage run -m py.test {posargs}
     coverage report
 
 [testenv:pypy]
 # temporarily disable coverage testing on PyPy due to performance problems
-commands= py.test {posargs} {toxinidir}/test/
+commands= py.test {posargs}
 
 [testenv:lint]
 basepython=python3.7