Update fakefs requirement from ~> 2.5 to ~> 3.0

Updates the requirements on [fakefs](https://github.com/fakefs/fakefs) to permit the latest version.
- [Release notes](https://github.com/fakefs/fakefs/releases)
- [Changelog](https://github.com/fakefs/fakefs/blob/master/CHANGELOG.md)
- [Commits](fakefs/fakefs@v2.5.0...v3.0.0)

---
updated-dependencies:
- dependency-name: fakefs
  dependency-type: direct:development
...

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

asciidoctor-doctest.gemspec

@@ -36,7 +36,7 @@ A tool for end-to-end testing of Asciidoctor backends based on comparing of text
   s.add_development_dependency 'thread_safe', '~> 0.3'
 
   # unit tests
-  s.add_development_dependency 'fakefs', '~> 2.5'
+  s.add_development_dependency 'fakefs', '~> 3.0'
   s.add_development_dependency 'rspec', '~> 3.1'
   s.add_development_dependency 'rspec-collection_matchers', '~> 1.1'