build pb on python 3.9

[agramfort]

we started to have build failures on python 3.9 in benchopt:

https://github.com/benchopt/benchmark_lasso/pull/18/checks?check_run_id=2128864808

thanks for taking a lot.

[StrikerRUS]

Hello @agramfort !
You can use the following workaround while the hot-fix PR is not merged:

https://github.com/BayesWitnesses/m2cgen/blob/6a7d67ff55f7a916981ab0d767f97eec404dfa58/requirements-test.txt#L6
https://github.com/benchopt/benchmark_lasso/blob/ca6f732f7ca370d27170145b971a77d64ef175e7/solvers/lightning.py#L16