Disable py.test --doctest-modules.

As we introduced an py.test autouse fixture, we cannot currently also
run doctest-modules due to a bug in py.test (2.8.1, 2.8.2):

   https://github.com/pytest-dev/pytest/issues/1057

Looks like a fix is planned for 2.8.3 release. Until then we have to
disable doctests in modules.

setup.cfg

@@ -4,8 +4,9 @@ dev = develop easy_install diceware[tests]
 docs = develop easy_install diceware[docs]
 
 [pytest]
-addopts = --doctest-modules
-          --doctest-glob='*.rst'
+# py.test 2.8.2 breaks with --doctest-modules
+# See https://github.com/pytest-dev/pytest/issues/1057
+addopts = --doctest-glob='*.rst'
           diceware tests docs README.rst
 
 [bdist_wheel]