Merge Styles into CSS... extension fails in flatpak, missing 'cssselect' module

Summary:

In the Inkscape flatpak (both stable and beta), choosing the command Extensions > Stylesheet > Merge Styles into CSS... fails. This is probably, like issue #2886 (moved), because the python cssselect module isn't included.

Steps to reproduce:

• open Inkscape
• create new document
• choose Extensions > Stylesheet > Merge Styles into CSS...
• enter a New Class Name, e.g. "test"
• click [Apply]

What happened?

A dialog appears

Inkscape has received additional data... The script did not return an error...

... File "/app/share/inkscape/extensions/inkex/css.py", line 27, in import cssselect ModuleNotFoundError: No module named 'cssselect'

What should have happened?

No error.

Presumably, the Flatpak manifest here it is for flathub stable needs to include a python3-cssselect module.

Sample attachments:

Version info

Inkscape 1.2-beta (1b65182ce9, 2022-04-05)

    GLib version:     2.72.0
    GTK version:      3.24.33
    glibmm version:   2.66.1
    gtkmm version:    3.24.5
    libxml2 version:  2.9.12
    libxslt version:  1.1.34
    Cairo version:    1.16.0
    Pango version:    1.50.6
    HarfBuzz version: 3.0.0

    OS version:       GNOME 42 (Flatpak runtime)