deps: bump pypdf past three advisories

pypdf 6.15.0 is affected by GHSA-763m-79hh-57f2 and GHSA-23w6-3w8w-8484
(fixed in 6.16.1) and GHSA-jp53-mhqp-8xcg (6.16.0). It arrives through
browser-use, which asks only for pypdf>=5.7.0.

The constraint floors it so a later re-resolve cannot drop back.