Bump mistune from 0.8.1 to 2.0.3

Bumps [mistune](https://github.com/lepture/mistune) from 0.8.1 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@v0.8.1...v2.0.3)

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

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

Author: dependabot[bot]

requirements.txt

@@ -1 +1 @@
-mistune==0.8.1
+mistune==2.0.3

setup.py

@@ -10,7 +10,7 @@ def read(f):
 
 
 install_requires = [
-    'mistune <= 1',
+    'mistune < 3',
 ]
 
 tests_require = [