Trac #34940: sagemath_doc_html depends on fpylll (for building)

but this dependency is not listed. Thus, one can see a build error (I
just run into it)

URL: https://trac.sagemath.org/34940
Reported by: dimpase
Ticket author(s): Dima Pasechnik
Reviewer(s): Matthias Koeppe

Author: Release Manager