Update blessed requirement from ~=1.18.0 to ~=1.20.0

Updates the requirements on [blessed](https://github.com/jquast/blessed) to permit the latest version.
- [Release notes](https://github.com/jquast/blessed/releases)
- [Changelog](https://github.com/jquast/blessed/blob/master/docs/history.rst)
- [Commits](jquast/blessed@1.18.0...1.20.0)

---
updated-dependencies:
- dependency-name: blessed
  dependency-type: direct:production
...

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

requirements.txt

@@ -1,4 +1,4 @@
 setuptools~=68.1.2
-blessed~=1.18.0
+blessed~=1.20.0
 cmd2~=2.4.3
 pygame~=2.5.1