Bump urwid from 2.6.12 to 2.6.15

Bumps [urwid](https://github.com/urwid/urwid) from 2.6.12 to 2.6.15.
- [Release notes](https://github.com/urwid/urwid/releases)
- [Changelog](https://github.com/urwid/urwid/blob/master/docs/changelog.rst)
- [Commits](urwid/urwid@2.6.12...2.6.15)

---
updated-dependencies:
- dependency-name: urwid
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

Signed-off-by: dependabot[bot] <[email protected]>