No, you probably don't need to put the BSD reference into the library, but you should.
When you re-release the BSD component within your GPLv3 package you are re-licensing the BSD component as well. At that point, the only license text that's required is the GPLv3 one.
The GPLv3 license contains the same or similar clauses as the BSD license. In particular, section 4 requires maintaining the copyright notices. Sections 15, 16, and 17 handle the disclaimer of warranty. And the section "How to apply these terms" goes into the details on propagating the conditions of the license.
However, as a courtesy to the original authors on the BSD licensed component, you should provide full attribution and a copy of the BSD license. Effectively you're saying "If you want to use this component from my distribution, then it's under GPLv3. However, you can access this component from its source under a BSD license."
Some supplementing points.
It means that the other license and the GNU GPL are compatible; you can combine code
released under the other license with code released under the GNU GPL in one larger
program.
All GNU GPL versions permit such combinations privately; they also permit distribution
of such combinations provided the combination is released under the same GNU GPL
version. The other license is compatible with the GPL if it permits this too.
The very last sentence (The other license is compatible...) in that quoted section along with the preceding combination clause (they also permit distribution...) is your permission (and requirement) to distribute the combination of your project + lxml as GPLv3.
The BSD license (without the advertising clause) is on FSF's list of compatible licenses. Specifically here. This is the same license the lxml team is using.
- Whether you provide a copy of lxml source is kind of a moot point.
Your program won't work without their program, so you can't really get around dragging their program into yours from a licensing point of view as far as the GPL is concerned. It's a "known" circumvention trick that isn't allowed. Fortunately, the licenses are compatible. Because you are choosing to release under the GPLv3 then you're re-licensing the lxml project as expressed / instantiated in your project. Essentially, you've forked the license on lxml but only for within your project.
The lxml team can continue to update and release their code under modified BSD, but the instantiation you use and any modifications you make to it within your project are considered as GPLv3. Note that you could still contribute directly to lxml and then they can release your contribution as BSD. But you can't do that through your GPL project.
