Only include taskcluster.async on Python 3

setup.py

@@ -9,6 +9,10 @@
 # that script works if you want to change this
 VERSION = '1.3.4'
 
+packages = [
+    'taskcluster',
+]
+
 tests_require = [
     'nose==1.3.7',
     'nose-exclude==0.5.0',
@@ -64,6 +68,7 @@ def run_tests(self):
 elif sys.version_info[:2] < (3, 5):
     raise Exception('This library does not support Python 3 versions below 3.5')
 elif sys.version_info[:2] >= (3, 5):
+    packages.append('taskcluster.async')
     install_requires.extend([
         'aiohttp>=2.0.0,<3',
         'async_timeout>=1.2.0,<2',
@@ -77,7 +82,7 @@ def run_tests(self):
         author='John Ford',
         author_email='jhford@mozilla.com',
         url='https://github.com/taskcluster/taskcluster-client.py',
-        packages=['taskcluster', 'taskcluster.async'],
+        packages=packages,
         install_requires=install_requires,
         test_suite="nose.collector",
         tests_require=tests_require,