Bump pyyaml from 3.12 to 5.1

Bumps [pyyaml](https://github.com/yaml/pyyaml) from 3.12 to 5.1.
- [Release notes](https://github.com/yaml/pyyaml/releases)
- [Changelog](https://github.com/yaml/pyyaml/blob/master/CHANGES)
- [Commits](https://github.com/yaml/pyyaml/compare/3.12...5.1)

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

optional-requirements.txt

@@ -8,7 +8,7 @@ uritemplate==3.0.0
 pyasn1-modules==0.0.8
 pyasn1==0.1.9
 PyDrive==1.3.1
-PyYAML==3.12
+PyYAML==5.1
 rsa==3.4.2
 six==1.10.0