`build/pkgs/pari`: Update to 2.15.5 #16194

Job	Run time
test-new	48m 56s
test-mod (sagemath_categories-check)	4m 48s
test-long (src/sage/[a-f]*)	32m 42s
test-long (src/sage/[g-o]*)	37m 1s
test-long (src/sage/[p-z]*)	33m 49s
test-long (src/doc src/sage_docbuild src/sage_setup)	12m 12s
coverage-report	56m 24s
	3h 45m 52s

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