Bug#919774: add dependency on python3-gi-cairo

[Jordi Pujol]

Package: meld
Version: 3.20.0-1
Severity: normal

This version doesn't show nothing in the differences window if this
package is not installed
Also, we can see this message in the terminal:
TypeError: Couldn't find foreign struct converter for 'cairo.Context'

Solved:
apt install python3-gi-cairo

the dependency on python-gi-cairo implies also python3-gi, but the
opposite is not true.

Thanks,

Jordi Pujol