Bump mistune from 0.8.4 to 2.0.3 in /requirements

Bumps [mistune](https://github.com/lepture/mistune) from 0.8.4 to 2.0.3.
- [Release notes](https://github.com/lepture/mistune/releases)
- [Changelog](https://github.com/lepture/mistune/blob/master/docs/changes.rst)
- [Commits](lepture/mistune@v0.8.4...v2.0.3)

---
updated-dependencies:
- dependency-name: mistune
  dependency-type: direct:production
...

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

requirements/requirements-python2.7.txt

@@ -192,7 +192,7 @@ marshmallow-enum==1.5.1
 marshmallow-sqlalchemy==0.18.0
 marshmallow==2.19.5
 mccabe==0.6.1
-mistune==0.8.4
+mistune==2.0.3
 mock==3.0.5
 mongomock==3.19.0
 monotonic==1.5