[python] Specify python_requires

Indicate to install tools that the generated code won't work with Python
3.0.x, 3.1.x and 3.2.x (due to use of `u"foo"` string literals).

Closes #192 and #191, opened by Andreas Maier.

python/setup.py

@@ -78,5 +78,6 @@
       license="BSD-3-Clause",
       packages=['snowballstemmer'],
       package_dir={"snowballstemmer": "src/snowballstemmer"},
+      python_requires='!=3.0.*, !=3.1.*, !=3.2.*',
       classifiers = classifiers
 )