I have to say that looks scarily like the robinia seedlings which plague my existence (because neighbouring tree)).

Robinia pseudoacacia

Black locust is what they're telling you.

I guess I will have to leave it outdoors this winter and let nature take its course. I have no space for a sun loving tree. I do want to grow a wisteria in a medium sized pot as a sort of largish bonsai.

Black Locust suckers and produces groves. The frangrance of their flowers permeates the air. They produce some dangerous thorns, but the amount of thorns varries and some trees don't have any thorns. The thorns probably evolved as protection from megafauna. Hawthorns,Crataegus has same huge thorns. I think pseudoacacia is endemic to an area where it remained restricted. Humans started moving them around and it's very invasive.

Not to be contrary but maybe japanese scholar tree (sophora)? also a large tree but easily pruned.

It does looks very much like a black locust but also a tiny bit ’off’. A green stem would point more towards sophora, i think the locust tends to gray.

Black locusts are considered invasive in Maine and I have seen large groves of it. I get volunteers of it all over my yard. It looks pretty close, but not close enough for me to say it's the same as the stuff that plagues me.

I thought it sophora too based on google image search. The leaves are roundish, and a bit glossy and thick. May be i will try to turn it into a tabletop bonsai.

Agree w/katob/getgoing, at least that it doesn't look "right" for black locust. Leaf shape, texture, form, something....

It doesn't look like black locust to me, either, and I've been around them all of my life.

There is a pagoda tree (styphnolobium japonica) at the end of my road...which does not look like the OP at all...although I am looking at a fully mature tree and not a young sapling. Nevertheless, the shape of each leaflet is considerably more elongated. I couldn't speak to other sophora sp.