Use `azjezz/psl` instead of `beberlei/assert`

[mariosimao]

You might want to use azjezz/psl for improved type inference, in future: I've used beberlei/assert and webmozart/assert extensively over the years, but @azjezz's work is just so much better, long-term

Originally posted in https://github.com/mariosimao/notion-sdk-php/pull/6#discussion_r744172069

[azjezz]

for reference, you can look at https://github.com/Roave/BackwardCompatibilityCheck/pull/306 and https://github.com/laminas/automatic-releases/pull/138