Fix fields_desc type hint to allow Packet class references

[T3pp31]

Summary

• widen Packet.fields_desc type annotation to ClassVar[List[Union[AnyField, Type[Packet]]]]
• add # type: ignore at runtime usage sites where mypy cannot infer the post-metaclass type
• no change to build/dissect behavior

Closes #5018

Approach

Per reviewer feedback, the fix follows the original issue's suggestion: just
update the one typing that's incorrect and use # type: ignore where absolutely
necessary elsewhere. No helper function is added.

At runtime, Packet_metaclass.__new__ resolves Packet references to Field
instances before any instance method runs. The widened annotation causes mypy
to treat self.fields_desc iteration as possibly containing Type[Packet],
so # type: ignore is added at the 17 call sites where mypy reports errors.
Two backslash-continued lines were converted to parenthesized form to allow
# type: ignore on the continued expression.

Why no new UTScapy tests

Per CONTRIBUTING.md, this is a typing-only fix with no runtime behavior change:

• build/dissect/show paths are unchanged
• the Packet-in-fields_desc pattern is already exercised by existing tests, e.g.
  test/contrib/loraphy2wan.uts (LinkADRReq inlines DataRate_TXPower and
  Redundancy Packet classes in fields_desc)
• regression coverage for this change is provided by tox -e mypy and tox -e flake8

Test plan

• tox -e flake8
• tox -e mypy (Python 3.12, linux and win32)

[codecov]

Codecov Report

❌ Patch coverage is 95.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 80.21%. Comparing base (14cc726) to head (f031589).
⚠️ Report is 12 commits behind head on master.

Files with missing lines	Patch %	Lines
scapy/packet.py	95.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5020      +/-   ##
==========================================
- Coverage   80.34%   80.21%   -0.13%     
==========================================
  Files         386      387       +1     
  Lines       96012    96446     +434     
==========================================
+ Hits        77137    77366     +229     
- Misses      18875    19080     +205     

Files with missing lines	Coverage Δ
scapy/packet.py	84.70% <95.00%> (ø)

... and 19 files with indirect coverage changes

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[gpotter2]

Hi & thanks for the PR.
The fix provided in the original issue looked more correct: don't add a function, just update the one typing that's incorrect and use # type: ignore if absolutely necessary elsewhere.