Bump bleach from 3.1.3 to 3.3.0 in /backend

Bumps [bleach](https://github.com/mozilla/bleach) from 3.1.3 to 3.3.0.
- [Release notes](https://github.com/mozilla/bleach/releases)
- [Changelog](https://github.com/mozilla/bleach/blob/master/CHANGES)
- [Commits](mozilla/bleach@v3.1.3...v3.3.0)

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

backend/requirements.txt

@@ -5,7 +5,7 @@ astor==0.8.0
 attrs==19.3.0
 backcall==0.1.0
 bcrypt==3.1.7
-bleach==3.1.3
+bleach==3.3.0
 blessed==1.17.4
 bokeh==2.0.0
 botocore==1.14.17