Drop (or fix) POMDPGym.jl dependency

[zengmao]

I've made a fork of this repo with the POMDPGym.jl dependency stripped off, so that the rest of the package can be installed without error in the latest Julia 1.11:
https://github.com/zengmao/Crux.jl

POMDPGym.jl is apparently broken, or at least hard to install, in the latest Julia versions due to Python package management issues. If this can be fixed, or if an alternative interface with OpenAI Gym / Gymnasium can be found, it would be the best scenario. An alternative approach, as in my experiment with the forked repo, is to trim down the package to a pure Julia core.

[ancorso]

I'd be fine with this! @mossr thoughts? If @mossr is ok with this then go ahead and open a PR if you want.

[mossr]

Yeah I agree that we can lighten the POMDPGym dependency. I'm all for this.

[zengmao]

Looks like @mossr has already made commits to do this. Thanks! Since POMDPGym.jl has also been fixed, in principle it can be treated as Test-specific dependencies supported by Pkg.jl.