Stop installing tests

Add a missing exclude to find_packages() in order to stop it from
installing global-scope 'tests' package.

Fixes #64

Author: mgorny

setup.py

@@ -12,7 +12,7 @@
     description='Sphinx extension for Read the Docs overrides',
     install_requires=['requests', 'Jinja2>=2.9'],
     package_dir={'': '.'},
-    packages=find_packages('.'),
+    packages=find_packages('.', exclude=['tests']),
     long_description=codecs.open("README.rst", "r", "utf-8").read(),
     # trying to add files...
     include_package_data=True,