Well I finally found it in setup.py : GPL.
Well, that doesn't say which version of the GPL.
And IMHO you should include a copy of the GPL in a LICENCE.txt file.
It would probably be GPLv2. Mercurial itself, and I believe all of its extensions, are GPLv2, and I'm pretty sure there will be no relicensing to v3. So if hgsubversion ever wanted to be incorporated to the hg repository, it would have to be v2.
The license is the same as that of Mercurial (GPLv2, no relicensing to later versions) out of necessity.
Why are you making a debian package? This is *not* release quality, I really really don't want to deal with the support burden of a bunch of regular users just yet.