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

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.63%. Comparing base (5f60db0) to head (7edd8df).
⚠️ Report is 10 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #5123   +/-   ##
=======================================
  Coverage   80.63%   80.63%           
=======================================
  Files         390      390           
  Lines       96903    96918   +15     
=======================================
+ Hits        78135    78152   +17     
+ Misses      18768    18766    -2     

Files with missing lines	Coverage Δ
scapy/packet.py	84.80% <100.00%> (ø)

... and 6 files with indirect coverage changes

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[gpotter2]

This breaks mypy. You might need to add some more type hints or #ignore maybe. I don't know what would be the best

[jankesec]

Hey @gpotter2, good catch!

I dug into the mypy failures: since Packet_metaclass unpacks and flattens Type[Packet] into fields when the class is defined, self.fields_desc is strictly List[AnyField] once initialized. However, mypy was complaining across packet.py because it assumed self.fields_desc could still hold packet classes at runtime.

I've updated the internal loops in scapy/packet.py with cast(List[AnyField], ...) to make that guarantee explicit to mypy. I tested it locally against both the linux / win32 mypy runs and the UTScapy suite (fields.uts and regression.uts), and everything is green with 0 errors now.

Just pushed the update — let me know what you think!