Fix dependency on enum34 (#26)

## What is the goal of this PR?

Fix `client-python` on 3.4+

## What are the changes implemented in this PR?

`enum34` is incompatible with Python 3.4+. Instead, we replace it with `enum-compat` which a no-op on incompatible versions

BUILD

@@ -44,7 +44,7 @@ py_library(
         requirement("protobuf"),
         requirement("grpcio"),
         requirement("six"),
-        requirement("enum34"),
+        requirement("enum_compat"),
     ],
     visibility =["//visibility:public"]
 )
@@ -74,7 +74,7 @@ deploy_pip(
     author = "Grakn Labs",
     author_email = "[email protected]",
     license = "Apache-2.0",
-    install_requires=['grpcio==1.16.0', 'protobuf==3.6.1', 'six==1.11.0', 'enum34==1.1.6'],
+    install_requires=['grpcio==1.16.0', 'protobuf==3.6.1', 'six==1.11.0', 'enum-compat==0.0.2'],
     keywords = ["grakn", "database", "graph", "knowledgebase", "knowledge-engineering"],
     deployment_properties = "@graknlabs_grakn_core//:deployment.properties",
     description = "A Python client for Grakn.",

requirements.txt

@@ -2,5 +2,6 @@ grpcio==1.16.0
 protobuf==3.6.1
 six==1.11.0
 forbiddenfruit==0.1.2
-enum34==1.1.6
+# non-native Enum < 3.4 and Native Enum 3.4 onwards
+enum-compat==0.0.2