Bump bleach from 3.1.0 to 3.1.2

Bumps [bleach](https://github.com/mozilla/bleach) from 3.1.0 to 3.1.2.
- [Release notes](https://github.com/mozilla/bleach/releases)
- [Changelog](https://github.com/mozilla/bleach/blob/master/CHANGES)
- [Commits](https://github.com/mozilla/bleach/compare/v3.1.0...v3.1.2)

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

requirements_dev.txt

@@ -4,7 +4,7 @@ asn1crypto==0.24.0
 atomicwrites==1.3.0
 attrs==19.1.0
 Babel==2.6.0
-bleach==3.1.0
+bleach==3.1.2
 bumpversion==0.5.3
 certifi==2019.3.9
 cffi==1.12.2