* Fix ubuntu version to 22.04 in python test environment setup.

* Try using python 3.9, but keeping latest ubuntu version.

* Try keeping latest ubunut and python 3.8, but change to newer checkout/setup verions.

…inimizers (#841)

* enable (in)equality constraints for use with scipy.optimize.minimize interface; support a few more minimisers; redefine _minimizer_callback signature to support trust-constr; add quick minimiser tests; minor touchups

* rename nested functions so they're not attempted to be tested individually

* fix scipy global solvers' configurations with or without constraints as much as possible

* add example global min. (DE) config file with theta23 inequality constraint

* add commented-out example DE scipy min. test configured from file

* superficial

* Update discrete systematic README (Ultrasurface use)

The previous version described the use of scripts that aren't part of pisa:

* remove the pisa unrelated descriptions
* update the use instructions with specific config and pipeline example

* Implement PR review comments