Bump pyyaml from 5.3 to 5.3.1

Bumps [pyyaml](https://github.com/yaml/pyyaml) from 5.3 to 5.3.1.
- [Release notes](https://github.com/yaml/pyyaml/releases)
- [Changelog](https://github.com/yaml/pyyaml/blob/master/CHANGES)
- [Commits](yaml/pyyaml@5.3...5.3.1)

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

Author: dependabot-preview[bot]

dev-requirements.txt

@@ -1,6 +1,6 @@
 # We use dev-requirements.txt instead of just declaring the requirements here
 # because Read The Docs needs a requirements file.
-PyYAML==5.3
+PyYAML==5.3.1
 Pygments==2.6.1
 Sphinx-Substitution-Extensions==2020.2.21.0
 Sphinx==2.4.4