typing: allow Packet classes in Packet.fields_desc type hint

[jankesec]

Summary

This PR updates the type annotation of Packet.fields_desc to allow references to Packet classes (subclasses), resolving #5018.

Background

At runtime, Packet_metaclass.__new__ in scapy/base_classes.py explicitly resolves references to other Packet subclasses within fields_desc by inlining/flattening their fields:

if "fields_desc" in dct:
    current_fld = dct["fields_desc"]
    resolved_fld = []
    for fld_or_pkt in current_fld:
        if isinstance(fld_or_pkt, Packet_metaclass):
            for pkt_fld in fld_or_pkt.fields_desc:
                resolved_fld.append(pkt_fld)
        else:
            resolved_fld.append(fld_or_pkt)

However, Packet.fields_desc was typed only as ClassVar[List[AnyField]], causing static type checkers to raise typing errors when referencing other Packet classes in fields_desc.

Changes

• Updated Packet.fields_desc type annotation in scapy/packet.py from ClassVar[List[AnyField]] to ClassVar[List[Union[AnyField, Type[Packet]]]].
• Tested with UTScapy suite (fields.uts).

Fixes #5018