It's not mint.

Maybe a Senna obtusifolia.

might it be a tree of heaven seedling, or a locust seedling?

Tree of Heaven has pointy leaves and these are round. It's an invasive pest though so I would definitely pull one up if I identified it as such.

Locust, maybe. Do you have locust trees around?

Purple may have it! I had to Google it to find out what that was, but it sure looks like it. Apparently considered a noxious weed in some places.

A native legume. Host plant for several species of sulfur butterfly caterpillars. Poisonous to livestock, lots of seeds.

Yes, it does look like the seedlings of the Senna obtusifolia (though I've never heard the real name before now). 'Around here' we call it a coffee bean and since we have horses, we pull them up as fast as we can find them.

Never heard it called coffee. We call it sickle pod, bad weed.

My spouse just bought a foreclosure for investment and among the things still growing was something with leaves that look like the OP's picture. This one is a woody plant, 1- 2 inches in diameter and chin high. Cannot tell if it is a tree or a shrub as there are several off shoots from the base. It is growing in a quite shelter niche so the zone probably doesn't mean much.