Add typing_extensions dependency

Pipfile

@@ -11,7 +11,8 @@ requests = "*"
 sphinxcontrib-phpdomain = "*"
 sphinxcontrib-blockdiag = "*"
 sphinxcontrib-seqdiag = "*"
+typing-extensions = "*"
 
 [dev-packages]
 
-[requires]
+[requires]

Pipfile.lock

@@ -1,7 +1,7 @@
 {
     "_meta": {
         "hash": {
-            "sha256": "5440e8048a85195523941eef9df26b93db42a5c6a0a88f554fda93c2a65556f5"
+            "sha256": "b59e41d109798b5e65fafcbe3a04b6a70f5322dbe362114043d4c432abf3a6de"
         },
         "pipfile-spec": 6,
         "requires": {},
@@ -369,6 +369,14 @@
             "markers": "python_version >= '3.5'",
             "version": "==1.1.5"
         },
+        "typing-extensions": {
+            "hashes": [
+                "sha256:25642c956049920a5aa49edcdd6ab1e06d7e5d467fc00e0506c44ac86fbfca02",
+                "sha256:e6d2677a32f47fc7eb2795db1dd15c1f34eff616bcaf2cfb5e997f854fa1c4a6"
+            ],
+            "index": "pypi",
+            "version": "==4.3.0"
+        },
         "urllib3": {
             "hashes": [
                 "sha256:8298d6d56d39be0e3bc13c1c97d133f9b45d797169a0e11cdd0e0489d786f7ec",