chore: update sphinx requirement from ==2.* to ==3.*

Updates the requirements on [sphinx](https://github.com/sphinx-doc/sphinx) to permit the latest version.
- [Release notes](https://github.com/sphinx-doc/sphinx/releases)
- [Changelog](https://github.com/sphinx-doc/sphinx/blob/3.x/CHANGES)
- [Commits](sphinx-doc/sphinx@v2.0.0b1...v3.0.0)

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

Author: dependabot-preview[bot]

Diff for: requirements.txt

@@ -5,7 +5,7 @@ m2r==0.*
 pep8==1.*
 pycodestyle==2.*
 pytest-cov==2.*
-Sphinx==2.*
+Sphinx==3.*
 sphinxcontrib-log-cabinet==1.*
 sphinxcontrib-websupport==1.*
 twine==3.*