Update test-unit requirement from ~> 3.5.0 to ~> 3.6.0 (#252)

Updates the requirements on [test-unit](https://github.com/test-unit/test-unit) to permit the latest version.
- [Release notes](https://github.com/test-unit/test-unit/releases)
- [Commits](test-unit/test-unit@3.5.0...3.6.0)

---
updated-dependencies:
- dependency-name: test-unit
  dependency-type: direct:development
...

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

pygments.rb.gemspec

@@ -26,7 +26,7 @@ Gem::Specification.new do |s|
 
   s.add_development_dependency 'rake', '~> 13.0.0'
   s.add_development_dependency 'rubocop', '~> 1.28.2'
-  s.add_development_dependency 'test-unit', '~> 3.5.0'
+  s.add_development_dependency 'test-unit', '~> 3.6.0'
 
   s.files = `git ls-files -z`.split("\0").reject { |f| File.symlink?(f) }
 end