Use itsdangerous library to sign cookie

[sblondon]

This PR is about issue #6.

The previous serialization/deserialization is still in the code, until it will be removed in a another future release.

What do you think about it? Do you see improvements to add?
Perhaps the itsdangerous library version should be more restricted?

[davidism]

This message needs to be more exact. Something like "Unserializing using the old scheme. This is deprecated and the fallback will be removed in version 2.0. Ensure cookies are re-serialized using the new ItsDangerous scheme."

Should also use stacklevel=3 or whatever level makes the error show where in user code caused it.

[sblondon]

I updated this part. The new string is longer than required by black so I added a # noqa comment to ignore it. If you prefer, I can extract it in a variable to reformat it.
I don't think it's necessary because the _mac_unserialize() method will disappear in a future release. What is your opinion about it?

[davidism]

Is this code similar to Flask's use of ItsDangerous for the session cookie? Haven't had a chance to compare yet. If not, we should identify how it's different and why here.

[sblondon]

The current added code for serialize() and unserialize() methods are based on the previous implementations (which are located in this patch in _mac_serialize() and _mac_unserialize() methods).

SecureCookieSessionInterface() class use classes from itsdangerous library (BadSignature and URLSafeTimedSerializer) but don't have serialize() and unserialize() methods.

So I don't understand what needs to be compared. Could you give me some hints?