Bump pyyaml from 3.11 to 5.1

Bumps [pyyaml](https://github.com/yaml/pyyaml) from 3.11 to 5.1.
- [Release notes](https://github.com/yaml/pyyaml/releases)
- [Changelog](https://github.com/yaml/pyyaml/blob/master/CHANGES)
- [Commits](yaml/pyyaml@3.11...5.1)

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

Author: dependabot[bot]

requirements.txt

@@ -36,7 +36,7 @@ pyflakes==1.2.3
 Pygments==2.2.0
 python-telegram-bot==9.0.0
 pytz==2018.3
-PyYAML==3.11
+PyYAML==5.1
 simplegeneric==0.8.1
 six==1.11.0
 snowballstemmer==1.2.1

requirements_dev.txt

@@ -7,5 +7,5 @@ tox==2.3.1
 coverage==4.1
 Sphinx==1.4.8
 cryptography==1.7
-PyYAML==3.11
+PyYAML==5.1
 python-telegram-bot==9.0.0