BLD: Require enum34 for Python < 3.4

pandas-dev#22802 imports `enum`, which was added to the built-ins in Python 3.4. The `enum34` package backports `enum` to earlier Python versions, so this should be a dependency as long as Python 2.7 is still supported.

Author: effigies

setup.py

@@ -29,6 +29,7 @@ def is_platform_windows():
     'install_requires': [
         'python-dateutil >= 2.5.0',
         'pytz >= 2011k',
+        'enum34; python_version < "3.4"',
         'numpy >= {numpy_ver}'.format(numpy_ver=min_numpy_ver),
     ],
     'setup_requires': ['numpy >= {numpy_ver}'.format(numpy_ver=min_numpy_ver)],