use only python 3.6 for now

cf. https://github.com/pypa/pipenv/issues/1050

Pipfile

@@ -15,7 +15,7 @@ GitPython = "*"
 djangorestframework = "*"
 
 [requires]
-python_version = ">=3.6"
+python_version = "3.6"
 
 [pipenv]
 allow_prereleases = true

Pipfile.lock

 {
     "_meta": {
         "hash": {
-            "sha256": "8aa12687e2b98dae1d44072f51160fa0bb424048237baa197110c9de93173d07"
+            "sha256": "80cb0e469667c93e2ed5b06718c97e0fe10bfba3a193b8b3ec7d22ce27dfea7c"
         },
         "pipfile-spec": 6,
         "requires": {
-            "python_version": ">=3.6"
+            "python_version": "3.6"
         },
         "sources": [
             {
@@ -131,6 +131,7 @@
                 "sha256:a68ac5e15e76e7e5dd2b8f94007233e01effe3e50e8daddf69acfd81cb686baf",
                 "sha256:b5725a0bd4ba422ab0e66e89e030c806576753ea3ee08554382c14e685d117b5"
             ],
+            "markers": "python_version != '3.1.*' and python_version != '3.2.*' and python_version < '4' and python_version != '3.3.*' and python_version >= '2.6' and python_version != '3.0.*'",
             "version": "==1.23"
         }
     },