python: use KV1Writer for Python KV1 bindings to fix lifetime issues

[cueki]

Previously:

kv1 = kvpp.KV1(test1)[0]  # immediately GC'd
kv2 = kvpp.KV1(test2)[0]

print(kv1['$basetexture'].value)  # nothing

Now we use KV1Writer as the Python KV1 class. Each element owns its data, so it works correctly :D.

[craftablescience]

pip build isn't properly updated, very very sorry 😭