Remove KeyError handling in _Dendrogram.get_color_dict

[tipabu]

new_old_color_map is hardcoded, and default_colors has a minimal key set that includes all the values.

Drive-by: simplify the overlay of rgb_colorscale onto default_colors.

Closes #2806

Code PR

• [X] I have read through the contributing notes and understand the structure of the package. In particular, if my PR modifies code of plotly.graph_objects, my modifications concern the codegen files and not generated files.
• [ ] I have added tests (if submitting a new feature or correcting a bug) or modified existing tests.
• [ ] For a new feature, I have added documentation examples in an existing or new tutorial notebook (please see the doc checklist as well).
• [ ] I have added a CHANGELOG entry if fixing/changing/adding anything substantial.

[nicolaskruchten]

@nicholas-esterer can you please review this one?

[nicholas-esterer]

The docs don't build but I don't understand the error. Logging into the build on CI and looking at one of the failures, we see that the basic.tpl template is missing. Does anyone know why? Here's a bit more of the error:

  File "/home/circleci/project/doc/nb.tpl", line 1, in top-level template code
    {%- extends 'basic.tpl' -%}
jinja2.exceptions.TemplateNotFound: basic.tpl