Upgrade to Weblate 3.x

Originally created by @intrigeri on #16943 (Redmine)

Wrt. the dependencies and their status in Debian, see tails#10038 (comment 73928).

Related issues

• Related to #16994 (closed)
• Related to tails#17050 (closed)
• Related to #16183 (closed)
• Blocks tails#16221 (closed)
• Blocks tails#16974
• Blocks #17358 (closed)