️ DOWNGRADE: attrs~=19.3

 This is required by the current markdow-it-py, but can be remove when moving to myst-nb 0.9

setup.py

@@ -54,6 +54,9 @@ setup(
         "docutils>=0.15",
         "sphinx<3",
         "myst-nb~=0.8.0",
+        # this is required by the current markdow-it-py
+        # but can be remove when moving to myst-nb 0.9
+        "attrs~=19.3",
         "click",
         "setuptools",
         "nbformat",