Bump pyxel from 1.8.6 to 1.8.8 (#68)

Bumps [pyxel](https://github.com/kitao/pyxel) from 1.8.6 to 1.8.8.
- [Release notes](https://github.com/kitao/pyxel/releases)
- [Changelog](https://github.com/kitao/pyxel/blob/main/CHANGELOG.md)
- [Commits](kitao/pyxel@v1.8.6...v1.8.8)

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

Signed-off-by: dependabot[bot] <support@github.com>

Signed-off-by: dependabot[bot] <support@github.com>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

requirements.txt

@@ -1,4 +1,4 @@
 # Keep the dependencies pinned, so we
 # can control breaking changes.
 
-pyxel==1.8.6
+pyxel==1.8.8