Update requires-all.txt

Author: deepyaman

requires-all.txt

@@ -30,7 +30,8 @@ PyYAML>=5.4.1
 diskcache>=5.2.1
 multiprocess>=0.70.12
 psutil>=5.8.0
-Flask>=1.0.4,<2.1.0
+Flask>=1.0.4,<2.1.0  # Flask 2.1.0 dropped support for Python 3.6
+Werkzeug<2.1.0  # Werkzeug 2.1.0 dropped support for Python 3.6
 flask-compress
 plotly>=5.0.0
 dash_html_components==2.0.0