Add support for extracting timestamp from a Fernet token (#4229)
* Add API for retrieving the seconds-to-expiry for the token, given a TTL.
* Process PR feedback:
* Do compute the TTL, but just the age of the token. The caller
can decided what to do next.
* Factored out the HMAC signature verification to a separate function.
* Fixed a copy&paste mistake in the test cases
* Tests cleanup.
* `struct` no longer needed
* Document `def age()`
* typo in `age()` documentation
* token, not data
* remove test for TTL expiry that is already covered by the parameterized `test_invalid()`.
* let's call this extract_timestamp and just return timestamp
* review comments
* it's UNIX I know this
4 files changed
