Move pytest config from `setup.cfg` to `tox.ini`.

Use of ``[pytest]`` section in `setup.cfg` is deprecated while
the new section name ``[tool:pytest]`` is not recognized by older
`pytest` versions.

To stay compatible with older `pytest` and avoid warnings with
`pytest` 3.0+, we move the whole config to `tox.ini`, where still
the old section name is valid.

setup.cfg

@@ -3,9 +3,5 @@ distribute = register sdist upload
 dev = develop easy_install diceware[tests]
 docs = develop easy_install diceware[docs]
 
-[pytest]
-addopts = --doctest-modules --doctest-glob='*.rst'
-          diceware tests docs README.rst
-
 [bdist_wheel]
 universal = 1

tox.ini

+[pytest]
+addopts = --doctest-modules --doctest-glob='*.rst'
+          diceware tests docs README.rst
+
 [tox]
 envlist = flake8, pypy, py26, py27, py33, py34, py35, coverage