Bootstrap5 introduced "accordion-flush" html class, which basically removes borders and rounding from accordions.

This could be added by creating an AccordionFlush python class at bootstrap.py and an accordion-flush.html layout.

Alternatively, the Accordion python class could be modified in order to accept an optional "accordion-flush" css class on the template, since it's the only difference from a standard accordion. I think this solution might be the more elegant one.

I'll try to work it out and propose a PR