py-JWT: updated to 2.6.0

v2.6.0

Changed
~~~~~~~
- bump up cryptography >= 3.4.0
- Remove `types-cryptography` from `crypto` extra

Fixed
~~~~~
- Invalidate token on the exact second the token expires
- fix: version 2.5.0 heading typo

Added
~~~~~
- Adding validation for `issued_at` when `iat > (now + leeway)` as `ImmatureSignatureError`

(adam)