python-mode: add 'exit' and 'quit' to the list of block-enders

python-mode: add 'exit' and 'quit' to the list of block-enders

'exit()' and 'quit()' end the current codeflow, there can't be
any code past it, similarly to 'return', 'continue', etc.  So
when calculating indentation for the line next to 'exit()',
'os.exit()', 'os._exit()', 'sys.exit()' and 'quit()', decrease
the indentation level.
* /lisp/progmodes/python.el (python-rx): Add 'exit()', 'os.exit()',
'os._exit()', 'sys.exit()', 'quit' to the list of block-enders.
(Bug#75729)

(cherry picked from commit 0514619b2c944ae17fbf649496595c5e866ff785)