DOC: add minimum version of tornado to ensure asyncio integration

tacaswell · tacaswell · commit aad08b240335 · 2021-10-20T17:55:16.000-04:00
tornado 5 is from ~2018 hence is consistent with our support window policy.
diff --git a/doc/devel/dependencies.rst b/doc/devel/dependencies.rst
@@ -46,7 +46,7 @@ and the capabilities they provide.
 * wxPython_ (>= 4) [#]_: for the wx-based backends.
 * pycairo_ (>= 1.11.0) or cairocffi_ (>= 0.8): for the GTK and/or cairo-based
   backends.
-* Tornado_: for the WebAgg backend.
+* Tornado_ (>=5): for the WebAgg backend.
 
 .. _Tk: https://docs.python.org/3/library/tk.html
 .. _PyQt5: https://pypi.org/project/PyQt5/
