requirements: Upgrade urwid from ~=2.1.2 to ~2.6.15.

[zormit]

What does this PR do, and why?

Upgrading urwid version, because why not. (description to be refined :P)

Outstanding aspect(s)

• Waiting for feedback from CI (I only briefly checked whether the application still starts, but didn't fully test it with the upgraded requirement)

External discussion & connections

• Discussed in #zulip-terminal in urwid version

How did you test this?

• Manually - Behavioral changes
• Manually - Visual changes
• Adapting existing automated tests
• Adding automated tests for new behavior (or missing tests)
• Existing automated tests should already cover this (only a refactor of tested code)

Self-review checklist for each commit

• It is a minimal coherent idea
• It has a commit summary following the documented style (title & body)
• It has a commit summary describing the motivation and reasoning for the change
• It individually passes linting and tests
• It contains test additions for any new behavior
• It flows clearly from a previous branch commit, and/or prepares for the next commit