Update pygments.rb requirement from ~> 2.4.0 to ~> 3.0.0 (#477)

Updates the requirements on [pygments.rb](https://github.com/pygments/pygments.rb) to permit the latest version.
- [Release notes](https://github.com/pygments/pygments.rb/releases)
- [Changelog](https://github.com/pygments/pygments.rb/blob/master/CHANGELOG.adoc)
- [Commits](pygments/pygments.rb@v2.4.0...v3.0.0)

---
updated-dependencies:
- dependency-name: pygments.rb
  dependency-type: direct:development
...

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

asciidoctor-epub3.gemspec

@@ -32,7 +32,7 @@ Gem::Specification.new do |s|
   s.add_development_dependency 'asciidoctor-diagram', '~> 2.0'
   s.add_development_dependency 'asciimath', '~> 2.0'
   s.add_development_dependency 'coderay', '~> 1.1.0'
-  s.add_development_dependency 'pygments.rb', '~> 2.4.0'
+  s.add_development_dependency 'pygments.rb', '~> 3.0.0'
   s.add_development_dependency 'rake', '~> 13.2.0'
   s.add_development_dependency 'rouge', '~> 3.0'
   s.add_development_dependency 'rspec', '~> 3.13.0'