Updated doctests to Python 3.5 and automatically running with Tox.

I've also removed doctests from the obsolete rsa/_versionXXX.py files,
as those files aren't even compatible with Python 3.x anyway.
diff --git a/setup.py b/setup.py
index d46945b..428b14c 100755
--- a/setup.py
+++ b/setup.py
@@ -16,39 +16,40 @@
 
 from setuptools import setup
 
-setup(name='rsa',
-      version='3.3',
-      description='Pure-Python RSA implementation',
-      author='Sybren A. Stuvel',
-      author_email='sybren@stuvel.eu',
-      maintainer='Sybren A. Stuvel',
-      maintainer_email='sybren@stuvel.eu',
-      url='https://stuvel.eu/rsa',
-      packages=['rsa'],
-      license='ASL 2',
-      classifiers=[
-          'Development Status :: 5 - Production/Stable',
-          'Intended Audience :: Developers',
-          'Intended Audience :: Education',
-          'Intended Audience :: Information Technology',
-          'License :: OSI Approved :: Apache Software License',
-          'Operating System :: OS Independent',
-          'Programming Language :: Python',
-          'Programming Language :: Python :: 3',
-          'Topic :: Security :: Cryptography',
-      ],
-      install_requires=[
-          'pyasn1 >= 0.1.3',
-      ],
-      entry_points={'console_scripts': [
-          'pyrsa-priv2pub = rsa.util:private_to_public',
-          'pyrsa-keygen = rsa.cli:keygen',
-          'pyrsa-encrypt = rsa.cli:encrypt',
-          'pyrsa-decrypt = rsa.cli:decrypt',
-          'pyrsa-sign = rsa.cli:sign',
-          'pyrsa-verify = rsa.cli:verify',
-          'pyrsa-encrypt-bigfile = rsa.cli:encrypt_bigfile',
-          'pyrsa-decrypt-bigfile = rsa.cli:decrypt_bigfile',
-      ]},
+if __name__ == '__main__':
+    setup(name='rsa',
+          version='3.3',
+          description='Pure-Python RSA implementation',
+          author='Sybren A. Stuvel',
+          author_email='sybren@stuvel.eu',
+          maintainer='Sybren A. Stuvel',
+          maintainer_email='sybren@stuvel.eu',
+          url='https://stuvel.eu/rsa',
+          packages=['rsa'],
+          license='ASL 2',
+          classifiers=[
+              'Development Status :: 5 - Production/Stable',
+              'Intended Audience :: Developers',
+              'Intended Audience :: Education',
+              'Intended Audience :: Information Technology',
+              'License :: OSI Approved :: Apache Software License',
+              'Operating System :: OS Independent',
+              'Programming Language :: Python',
+              'Programming Language :: Python :: 3',
+              'Topic :: Security :: Cryptography',
+          ],
+          install_requires=[
+              'pyasn1 >= 0.1.3',
+          ],
+          entry_points={'console_scripts': [
+              'pyrsa-priv2pub = rsa.util:private_to_public',
+              'pyrsa-keygen = rsa.cli:keygen',
+              'pyrsa-encrypt = rsa.cli:encrypt',
+              'pyrsa-decrypt = rsa.cli:decrypt',
+              'pyrsa-sign = rsa.cli:sign',
+              'pyrsa-verify = rsa.cli:verify',
+              'pyrsa-encrypt-bigfile = rsa.cli:encrypt_bigfile',
+              'pyrsa-decrypt-bigfile = rsa.cli:decrypt_bigfile',
+          ]},
 
-      )
+          )