[Debian-science-sagemath] sagemath and Python 3

[François Bissey]

I adopted trac 27971 to eliminate that noise in sage-on-gentoo.

> On 6/09/2019, at 11:23 PM, Tobias Hansen <thansen at debian.org> wrote:
> 
> Another issue is that the build log is 25 MB big because the docbuild prints this message all over:
> 
> RemovedInSphinx20Warning: formatargspec() is now deprecated.  Please use sphinx.util.inspect.Signature instead.