Use kwarg for Theme filename (#5190)

There was a recent change on Bokeh branch-2.4 to make the json and filename args to Theme be kwarg-only, but this is incompatible with Dask current usage. I will revert the change in Bokeh to maintain current compatibility, but I am also submitting the change here to set the stage for eventually re-introducing kwarg-only at some point in the future.

Secondarily, I have questions about what to do about testing? This was not caught by the "downstream" tests that Bokeh runs. A simple no-op test that just performs the import would have caught this since the use of Theme is at module scope. It's probably a good idea to make sure all of the bokeh-related modules cleanly import. But all of the bokeh related tests still seem to be in the dask repo, not this one. Should those tests be moved here? Or should I open a separate PR against dask to add tests?

Edit: reverted on Bokeh side at bokeh/bokeh#11484

distributed/dashboard/components/scheduler.py

@@ -84,7 +84,9 @@
     )
 )
 
-BOKEH_THEME = Theme(os.path.join(os.path.dirname(__file__), "..", "theme.yaml"))
+BOKEH_THEME = Theme(
+    filename=os.path.join(os.path.dirname(__file__), "..", "theme.yaml")
+)
 TICKS_1024 = {"base": 1024, "mantissas": [1, 2, 4, 8, 16, 32, 64, 128, 256, 512]}
 XLABEL_ORIENTATION = -math.pi / 9  # slanted downwards 20 degrees
 

distributed/dashboard/components/worker.py

@@ -45,7 +45,9 @@
     )
 )
 
-BOKEH_THEME = Theme(os.path.join(os.path.dirname(__file__), "..", "theme.yaml"))
+BOKEH_THEME = Theme(
+    filename=os.path.join(os.path.dirname(__file__), "..", "theme.yaml")
+)
 
 template_variables = {"pages": ["status", "system", "profile", "crossfilter"]}
 

distributed/http/scheduler/missing_bokeh.py

@@ -6,9 +6,9 @@ class MissingBokeh(RequestHandler):
     def get(self):
         with log_errors():
             self.write(
-                "<p>Dask needs bokeh >= 0.13.0 for the dashboard.</p>"
-                "<p>Install with conda: conda install bokeh>=0.13.0</p>"
-                "<p>Install with pip: pip install bokeh>=0.13.0</p>"
+                "<p>Dask needs bokeh >= 1.0 for the dashboard.</p>"
+                "<p>Install with conda: conda install bokeh>=1.0</p>"
+                "<p>Install with pip: pip install bokeh>=1.0</p>"
             )