Add back cvxopt to extra deps now that Python 3.11 wheels are available

[lanctot]

Ref: cvxopt/cvxopt#228.

• Upgrade cvxopt to version 1.3.1
• cvxopt 1.3.1 now supports Python 3.11
• Re-enable tests that depended on cvxopt, that were recently disabled (see Changes planned for release of Ubuntu 23.04 (Lunar Lobster) / Python 3.11 support #1025).
• Set Julia version to 1.8 since release recent 1.9 release is breaking CxxWrap (thanks @findmyway!)

[lanctot]

Hello @findmyway,

I am getting an odd error in Julia CxxWrap compilation seemingly unrelated to this change: https://github.com/deepmind/open_spiel/actions/runs/4941971630/jobs/8843020459

I tried rerunning the job twice and it still produces this same error.

Any ideas? Should I increase the version number of CxxWrap?

[findmyway]

[email protected] just got released yesterday. I guess CxxWrap is not updated corresponding yet.

As a temporary fix, we may need to set Julia version to 1.8 in the github action explicitly for now.

- uses: julia-actions/setup-julia@v1
  with:
    version: 1.8