Fix URLSafeSerializer.dumps to always return str

[daniel7an]

Summary

URLSafeSerializerMixin.dumps now always returns str regardless of the underlying serializer.

Problem

When using a non-text serializer (e.g. msgpack), URLSafeSerializer.dumps() returns bytes despite the type annotation declaring str as the return type (since URLSafeSerializer extends Serializer[str]):

>>> import msgpack
>>> from itsdangerous import URLSafeSerializer
>>> s = URLSafeSerializer('secret', serializer=msgpack)
>>> type(s.dumps({'id': 123}))
<class 'bytes'>

Fix

Override dumps in URLSafeSerializerMixin to always decode the result to str. Since the payload is base64-encoded, it is always valid UTF-8 and can safely be decoded regardless of the underlying serializer.

This also applies to URLSafeTimedSerializer through the mixin.

Fixes #410