Correct type for json_encoder argument

[jdufresne]

Per recent upstream fix to typeshed, json.dumps() cls argument should be
optional type JSONEncoder.

python/typeshed@8e0d288

Fixes mypy error:

jwt/api_jws.py:102: error: Argument "cls" to "dumps" has incompatible type "Optional[Callable[..., Any]]"; expected "Optional[Type[JSONEncoder]]"
jwt/api_jwt.py:61: error: Argument "cls" to "dumps" has incompatible type "Optional[Callable[..., Any]]"; expected "Optional[Type[JSONEncoder]]"

[coveralls]

Pull Request Test Coverage Report for Build 68

• 2 of 2 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.02%) to 76.757%

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Totals
Change from base Build 62:	0.02%
Covered Lines:	2206
Relevant Lines:	2874

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💛 - Coveralls

[jpadilla]

Thanks!