Require python >= 3

Add python_requires and python3 only classifiers.
This avoid pip on python2 downloading a non python2 compatible package.

__pkginfo__.py

@@ -38,3 +38,9 @@ install_requires = [
     'lxml',
     'html2text',
     ]
+
+classifiers = [
+    "Programming Language :: Python",
+    "Programming Language :: Python :: 3",
+    "Programming Language :: Python :: 3 :: Only",
+]

setup.py

@@ -76,7 +76,9 @@ def install(**kwargs):
                  data_files = data_files,
                  ext_modules = ext_modules,
                  namespace_packages = [subpackage_of],
+                 python_requires='>=3',
                  install_requires = install_requires,
+                 classifiers=pkginfo.get('classifiers', []),
                  **kwargs
                  )