Ensure tool string lookup import is robust (#4308)

holoviews/plotting/bokeh/element.py

@@ -22,13 +22,9 @@
 from bokeh.models.tickers import (
     Ticker, BasicTicker, FixedTicker, LogTicker, MercatorTicker
 )
+from bokeh.models.tools import Tool
 from bokeh.models.widgets import Panel, Tabs
 
-try:
-    from bokeh.plotting.helpers import _known_tools as TOOLS_MAP
-except:
-    from bokeh.plotting._tools import TOOLS_MAP
-
 from ...core import DynamicMap, CompositeOverlay, Element, Dimension, Dataset
 from ...core.options import abbreviated_exception, SkipRendering
 from ...core import util
@@ -51,6 +47,16 @@
     compute_layout_properties, wrap_formatter, match_ax_type, remove_legend
 )
 
+if bokeh_version >= '2.0.1':
+    try:
+        TOOLS_MAP = Tool._known_aliases
+    except Exception:
+        TOOLS_MAP = TOOL_TYPES
+elif bokeh_version >= '2.0.0':
+    from bokeh.plotting._tools import TOOLS_MAP
+else:
+    from bokeh.plotting.helpers import _known_tools as TOOLS_MAP
+
 
 
 class ElementPlot(BokehPlot, GenericElementPlot):