Bump bleach from 2.1.1 to 3.3.0 in /docs

Bumps [bleach](https://github.com/mozilla/bleach) from 2.1.1 to 3.3.0.
- [Release notes](https://github.com/mozilla/bleach/releases)
- [Changelog](https://github.com/mozilla/bleach/blob/master/CHANGES)
- [Commits](mozilla/bleach@v2.1.1...v3.3.0)

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

Author: dependabot[bot]

docs/requirements.txt

@@ -3,7 +3,7 @@ appnope==0.1.0
 asn1crypto==0.22.0
 astroid==1.5.3
 Babel==2.6.0
-bleach==2.1.1
+bleach==3.3.0
 certifi==2018.4.16
 cffi==1.10.0
 chardet==3.0.4