Not compatible with mistune 2.x

[chriskuehl]

Apr 03 17:41:32 ariel docker[185072]: Traceback (most recent call last):
Apr 03 17:41:32 ariel docker[185072]:   File "/srv/fluffy/venv/lib/python3.9/site-packages/gunicorn/arbiter.py", line 557, in spawn_worker
Apr 03 17:41:32 ariel docker[185072]:     worker.init_process()
Apr 03 17:41:32 ariel docker[185072]:   File "/srv/fluffy/venv/lib/python3.9/site-packages/gunicorn/workers/base.py", line 126, in init_process
Apr 03 17:41:32 ariel docker[185072]:     self.load_wsgi()
Apr 03 17:41:32 ariel docker[185072]:   File "/srv/fluffy/venv/lib/python3.9/site-packages/gunicorn/workers/base.py", line 136, in load_wsgi
Apr 03 17:41:32 ariel docker[185072]:     self.wsgi = self.app.wsgi()
Apr 03 17:41:32 ariel docker[185072]:   File "/srv/fluffy/venv/lib/python3.9/site-packages/gunicorn/app/base.py", line 67, in wsgi
Apr 03 17:41:32 ariel docker[185072]:     self.callable = self.load()
Apr 03 17:41:32 ariel docker[185072]:   File "/srv/fluffy/venv/lib/python3.9/site-packages/gunicorn/app/wsgiapp.py", line 65, in load
Apr 03 17:41:32 ariel docker[185072]:     return self.load_wsgiapp()
Apr 03 17:41:32 ariel docker[185072]:   File "/srv/fluffy/venv/lib/python3.9/site-packages/gunicorn/app/wsgiapp.py", line 52, in load_wsgiapp
Apr 03 17:41:32 ariel docker[185072]:     return util.import_app(self.app_uri)
Apr 03 17:41:32 ariel docker[185072]:   File "/srv/fluffy/venv/lib/python3.9/site-packages/gunicorn/util.py", line 357, in import_app
Apr 03 17:41:32 ariel docker[185072]:     __import__(module)
Apr 03 17:41:32 ariel docker[185072]:   File "/srv/fluffy/venv/lib/python3.9/site-packages/fluffy/run_prod.py", line 3, in <module>
Apr 03 17:41:32 ariel docker[185072]:     from fluffy.run import app  # noqa: E402 F401
Apr 03 17:41:32 ariel docker[185072]:   File "/srv/fluffy/venv/lib/python3.9/site-packages/fluffy/run.py", line 10, in <module>
Apr 03 17:41:32 ariel docker[185072]:     import fluffy.component.markdown  # noreorder # noqa
Apr 03 17:41:32 ariel docker[185072]:   File "/srv/fluffy/venv/lib/python3.9/site-packages/fluffy/component/markdown.py", line 47, in <module>
Apr 03 17:41:32 ariel docker[185072]:     mistune.Renderer,
Apr 03 17:41:32 ariel docker[185072]: AttributeError: module 'mistune' has no attribute 'Renderer'