Preserve requirements.txt

Such kind of hard pins should be avoided, but I'll leave that to
maintainers of web_widget_mpld3.

requirements.txt

@@ -1,6 +1,5 @@
-# web_widget_bokeh_chart
+# generated from manifests external_dependencies
 bokeh==1.1.0
-# web_widget_mpld3_chart
 matplotlib==3.0.3; python_version < '3.7'
 matplotlib==3.4.1; python_version >= '3.7'
 mpld3==0.3

setup/web_widget_mpld3_chart/setup.py

@@ -2,5 +2,15 @@ import setuptools
 
 setuptools.setup(
     setup_requires=['setuptools-odoo'],
-    odoo_addon=True,
+    odoo_addon={
+        "external_dependencies_override": {
+            "python": {
+                "mpld3": [
+                    "matplotlib==3.0.3; python_version < '3.7'",
+                    "matplotlib==3.4.1; python_version >= '3.7'",
+                    "mpld3==0.3",
+                ],
+            },
+        },
+    },
 )