kvpp: python fixes

[cueki]

While testing the python bindings I ran into a few weird parsing bugs, as well as some nanobind warnings.

This is basically what happens:

from pathlib import Path
from sourcepp import kvpp

LOC = Path("Deadlock/game/core/resource/valve_french.txt")
PCF = Path("tf/particles/eyeboss.pcf")
VDF = Path("Steam/userdata/<id>/config/shortcuts.vdf")

kv = kvpp.KV1(LOC.read_text("utf-8"))
kv[0].key                                                     # '\ufeff"lang"'
kv.get_child("lang", 0)                                       # not found
kvpp.KV1Writer(LOC.read_text("utf-8")).bake().splitlines()[0] # '"\ufeff"lang""'
                                            
kv.get_child("lang") # TypeError: get_child(): incompatible function arguments

kvpp.KV1Binary(VDF.read_bytes()).add_child("new") # TypeError: add_child(): incompatible function arguments

dmx = kvpp.DMX(PCF.read_bytes())
kv1b = kvpp.KV1Binary(VDF.read_bytes()) 
# nanobind: leaked 2 instances!

Fixes:

• KV1/KV1Writer skip a leading UTF-8 BOM
• get_child defaults n to 0
• add_child/add_attribute accept their None default (.none())
• constructors that take bytes no longer return new(self), which leaked every instance

[craftablescience]

Oops. thanks!