struct: Python 3.14 complex formats F and D are unsupported #8800

Feature

Python 3.14 added the F and D format codes for packing and unpacking
complex numbers. RustPython rejects both as invalid format characters.

Reproduced on main at 15c9bdd48:

import struct

for fmt in (">F", ">D"):
    try:
        data = struct.pack(fmt, 1 + 2j)
        print(fmt, len(data), struct.unpack(fmt, data))
    except struct.error as exc:
        print(fmt, "struct.error:", exc)

CPython 3.14.7:

>F 8 ((1+2j),)
>D 16 ((1+2j),)

RustPython:

>F struct.error: bad char in struct format
>D struct.error: bad char in struct format

The existing test_struct.StructTest.test_c_complex_round_trip is marked
as an expected failure.

Python Documentation or reference to CPython source code

https://docs.python.org/3/library/struct.html#format-characters