Add typing extensions to requirements.

requirements.in/base.txt

@@ -2,4 +2,5 @@ django
 msgpack
 pynacl
 fastapi
+typing_extensions
 uvicorn[standard]

requirements.txt

@@ -21,6 +21,7 @@ pyyaml==5.3.1             # via uvicorn
 six==1.15.0               # via pynacl
 sqlparse==0.4.1           # via django
 starlette==0.13.6         # via fastapi
+typing-extensions==3.7.4.3  # via -r requirements.in/base.txt
 uvicorn[standard]==0.13.2  # via -r requirements.in/base.txt
 uvloop==0.14.0            # via uvicorn
 watchgod==0.6             # via uvicorn