Bug#822586: cherrytree: depends on python-gtksourceview2 which is deprecated

[Kurobeats .]

python-appindicator has been removed as a dependency in commit
112c786f42835142a1b4ca0a47c6d351e26dc406

https://github.com/giuspen/cherrytree/commit/112c786f42835142a1b4ca0a47c6d351e26dc406

kuro.