Bump pyyaml from 5.3.1 to 5.4

Bumps [pyyaml](https://github.com/yaml/pyyaml) from 5.3.1 to 5.4.
- [Release notes](https://github.com/yaml/pyyaml/releases)
- [Changelog](https://github.com/yaml/pyyaml/blob/master/CHANGES)
- [Commits](yaml/pyyaml@5.3.1...5.4)

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

Author: dependabot-preview[bot]

dev-requirements.txt

@@ -1,7 +1,7 @@
 # We use dev-requirements.txt instead of just declaring the requirements in
 # the setup function because Read The Docs needs a requirements file.
 black==20.8b1
-PyYAML==5.3.1
+PyYAML==5.4
 Pygments==2.7.4
 Sphinx-Substitution-Extensions==2020.9.30.0
 Sphinx==3.4.3