Merge pull request #762 from MetaCell/dependabot/pip/tornado-6.3.3

Bump tornado from 6.1.0 to 6.3.3

Author: enicolasgomez

requirements.txt

@@ -91,7 +91,7 @@ scipy==1.4.1
 Send2Trash==1.5.0
 terminado==0.8.3
 testpath==0.4.4
-tornado==6.1.0
+tornado==6.3.3
 traitlets==4.3.3
 typing-extensions==3.7.4.2
 urllib3==1.25.9