Key rotate

[lepture]

This change is designed for pallets/flask#1574

With this change, Signer and Serializer can accept a list of secret keys. For instance,

1. In 2019/09, we are using a as the secret key
2. In 2019/10, we are going to use b as the secret key

For 2019/09, we will use Serializer(['a']), and later in 10, we will use Serializer(['a', 'b']). Now the old dumped values can still be loaded, because a is in secret keys. But it will dump new value with secret key b in 2019/10.

Later, we can remove a from the secret keys, since it is not used anymore. Maybe in 2019/11.

[lepture]

Documentation is still missing. This should be a 1.2.0 release. I'm not sure if the parameter secret_key should be renamed to secret_keys.

[ThiefMaster]

I think removing secret_key breaks backwards compatibility. This could be avoided by making it a property returning self.secret_keys[0].

[davidism]

I'll write some documentation and push it before merging. The most important part seems to be documenting that the secret_key list is in order of oldest to newest key. This makes sense (use .append to add a new key), but my first instinct was that the first item was the first key.

[davidism]

I think you removed secret_key= so it wasn't implied to be a named parameter? After thinking about it, I think it should remain secret_key, as the common use case is probably to use one key rather than rotation. Internally the name secret_keys is fine, and I'll ad a secret_key property as ThiefMaster mentioned.

[davidism]

Ended up making significant changes to the docs, so I'm going to make a separate PR for that.