Bump sphinx-substitution-extensions from 2019.12.28.0 to 2020.2.21.0

Bumps [sphinx-substitution-extensions](https://github.com/adamtheturtle/sphinx-substitution-extensions) from 2019.12.28.0 to 2020.2.21.0.
- [Release notes](https://github.com/adamtheturtle/sphinx-substitution-extensions/releases)
- [Changelog](https://github.com/adamtheturtle/sphinx-substitution-extensions/blob/master/CHANGELOG.rst)
- [Commits](adamtheturtle/sphinx-substitution-extensions@2019.12.28.0...2020.02.21.0)

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

Author: dependabot-preview[bot]

dev-requirements.txt

@@ -2,7 +2,7 @@
 # because Read The Docs needs a requirements file.
 PyYAML==5.3
 Pygments==2.5.2
-Sphinx-Substitution-Extensions==2019.12.28.0
+Sphinx-Substitution-Extensions==2020.2.21.0
 Sphinx==2.4.2
 VWS-Python-Mock==2019.12.27.0
 VWS-Test-Fixtures==2019.12.13.1