gnu: python-bokeh: Use Tornado 6.

* gnu/packages/python-xyz.scm (python-bokeh)[propagated-inputs]: Replace
python-tornado with python-tornado-6.

gnu/packages/python-xyz.scm

@@ -16408,7 +16408,7 @@ enhancements to optimization and data fitting problems.")
            python-packaging
            python-pillow
            python-pyyaml
-           python-tornado
+           python-tornado-6
            python-typing-extensions))
     (native-inputs
      (list python-beautifulsoup4