Use enum34 backport when Enum is not available

Author: Nabellaleen

setup.py

@@ -10,6 +10,16 @@
         ast.literal_eval(_version_re.search(f.read().decode("utf-8")).group(1))
     )
 
+requirements = [
+    "six>=1.10.0",
+    "graphene>=2.1.3",
+    "SQLAlchemy",
+    "singledispatch>=3.4.0.3",
+]
+try:
+    import Enum
+except ImportError:
+    requirements.append("enum34 >= 1.1.6")
 
 setup(
     name="graphene-sqlalchemy",
@@ -36,11 +46,6 @@
     ],
     keywords="api graphql protocol rest relay graphene",
     packages=find_packages(exclude=["tests"]),
-    install_requires=[
-        "six>=1.10.0",
-        "graphene>=2.1.3",
-        "SQLAlchemy",
-        "singledispatch>=3.4.0.3",
-    ],
+    install_requires=requirements,
     tests_require=["pytest>=2.7.2", "mock", "sqlalchemy_utils"],
 )