Fix #1871

For some reason Py3.6.4 fails to install dash without explicitly pinning importlib-metadata. Hopefully this will help!

Author: alexcjohnson

requires-install.txt

@@ -4,3 +4,4 @@ plotly>=5.0.0
 dash_html_components==2.0.0
 dash_core_components==2.0.0
 dash_table==5.0.0
+importlib-metadata==4.8.3;python_version<"3.7"