Use explicit Optional when default = None

[rekyungmin]

Implicit optional is not PEP484 compliant.

This is no longer the recommended behavior. Type checkers should move towards requiring the optional type to be made explicit.

Also Mypy will error on it in the future: python/mypy#9091

Expected Result

Use explicit Optional when default = None

Actual Result

Some implicit optionals are used.

Reproduction Steps

$ mypy --no-implicit-optional jwt

mypy fails.