Remove protocol < 3 support

[mykaul]

Just as was done for the GoCQL driver (scylladb/gocql#457 and others), we should remove ancient protocol support. It's not such a big deal, but there's always an 'if' in the serialization and so forth.
Example:

class _SimpleParameterizedType(_ParameterizedType):
    @classmethod
    def deserialize_safe(cls, byts, protocol_version):
        subtype, = cls.subtypes
        if protocol_version >= 3:
            unpack = int32_unpack
            length = 4
        else:
            unpack = uint16_unpack
            length = 2

Or:

    def serialize_safe(cls, items, protocol_version):
        if isinstance(items, str):
            raise TypeError("Received a string for a type that expects a sequence")

        subtype, = cls.subtypes
        pack = int32_pack if protocol_version >= 3 else uint16_pack