Don't install enum34 when it's not required

Having enum34 breaks Python 3.6 in some cases.

h/t https://hynek.me/articles/conditional-python-dependencies/

enumfields/enums.py

@@ -1,7 +1,11 @@
 import inspect
-from enum import Enum as BaseEnum
-from enum import EnumMeta as BaseEnumMeta
-from enum import _EnumDict
+
+try:
+    from enum import Enum as BaseEnum
+    from enum import EnumMeta as BaseEnumMeta
+    from enum import _EnumDict
+except ImportError:  # pragma: no cover
+    raise ImportError('Missing the enum module. Please install enum34.')
 
 from django.utils.encoding import python_2_unicode_compatible
 

setup.py

@@ -1,10 +1,11 @@
 #!/usr/bin/env python
 
 import os
-from setuptools import setup, find_packages
-from setuptools.command.test import test as TestCommand
 import sys
 
+from setuptools import find_packages, setup
+from setuptools.command.test import test as TestCommand
+
 
 def read(fname):
     return open(os.path.join(os.path.dirname(__file__), fname)).read()
@@ -25,11 +26,6 @@ def run_tests(self):
         errno = pytest.main(self.test_args)
         sys.exit(errno)
 
-install_requires = ['six']
-try:
-    import enum
-except ImportError:
-    install_requires.append('enum34')
 
 setup(
     name='django-enumfields',
@@ -57,10 +53,13 @@ def run_tests(self):
         "Programming Language :: Python :: 3.5",
         'Topic :: Internet :: WWW/HTTP',
     ],
-    install_requires=install_requires,
+    install_requires=['six'],
     tests_require=[
         'pytest-django<3.0',
         'Django',
     ],
+    extras_require={
+        ':python_version<"3.4"': ['enum34'],
+    },
     cmdclass={'test': PyTest},
 )