* 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

* add rotate method

* add some more tests for the failure modes

* start adding some documentation for the rotate method

* operate on a single token at a time, leave lists to the caller

* add versionadded
add versionadded, drop rotate from class doctest

* give rotate a doctest

* single level, not aligned

* add changelog for mf.rotate

* show that, once rotated, the old fernet instance can no longer decrypt the token

* add the instead of just the how

* update docs to reflect removal of ttl from rotate

* update tests

* refactor internal methods so that we can extract the timestamp

* implement rotate

* update wordlist (case sensitive?)

* lints

* consistent naming

* get_token_data/get_unverified_token_data -> better name

* doc changes

* use the static method, do not treat as imethod

* move up to MultiFernet docs

* add to authors

* alter wording

* monkeypatch time to make it less possible for the test to pass simply due to calls occuring in less than one second

* set the time after encryption to make sure that the time is preserved as part of re-encryption

Previously if the token claimed to have been generated more than 60
seconds in the future we would raise InvalidToken even if ttl was set
to None.

Fixes #1209

This way is more extensible and requires less maintaince