v3.0.0

• The serialization API has been reworked. Instead of consuming a serializer and deserializer as separate arguments, client objects now expect an argument serde to be an object which implements serialize and deserialize as methods. (serialize and deserialize are still supported but considered deprecated.)
• Validate integer inputs for expire, delay, incr, decr, and memlimit -- non-integer values now raise MemcacheIllegalInputError
• Validate inputs for cas -- values which are not integers or strings of 0-9 now raise MemcacheIllegalInputError
• Add prepend and append support to MockMemcacheClient.
• Add the touch method to HashClient.
• Added official support for Python 3.8.