Bump snowballstemmer from 1.2.1 to 1.9.0

Bumps [snowballstemmer](https://github.com/snowballstem/snowball) from 1.2.1 to 1.9.0.
- [Release notes](https://github.com/snowballstem/snowball/releases)
- [Commits](https://github.com/snowballstem/snowball/commits)

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

Author: dependabot-preview[bot]

docs-requirements.txt

@@ -26,7 +26,7 @@ pytz==2019.1              # via babel
 requests==2.22.0          # via sphinx
 six==1.12.0               # via packaging
 sniffio==1.1.0
-snowballstemmer==1.2.1    # via sphinx
+snowballstemmer==1.9.0    # via sphinx
 sortedcontainers==2.1.0
 sphinx-rtd-theme==0.4.3
 sphinx==2.1.2