DID token validate fails open on non-finite ext/nbf/iat

[SashaMIT]

Token.validate compares ext and nbf with > / <. Python json.loads accepts NaN and Infinity by default. Those comparisons are always false, so ext: Infinity never expires and nbf: NaN never blocks use. A string such as "never" raises TypeError instead of DIDTokenMalformed.

Language-split of magiclabs/magic-admin-js#156.

Happy to open a PR that fail-closes on non-finite iat / ext / nbf.