Bug#944544: sagemath: prompt_toolkit 2.x support

[Gordon Ball]

Source: sagemath
Severity: normal

sagemath depends on python3-prompt-toolkit, for which a transition has
been opened to upgrade to the 1.x series to 2.x (see #944227).

The dependency in d/control for sagemath is unversioned, but looking at
https://git.sagemath.org/sage.git/tree/build/pkgs/prompt_toolkit/package-version.txt
it looks like it supports 1.x; unclear whether there is 2.x support, or
the dependency is transitive with the version of ipython etc used.

(sagemath also has a residual dependency on python-prompt-toolkit, but
it looks like it's only some no-longer-built architectures)