Bokeh 3 - Fix tests

[hoxbro]

Fixes the tests

• Update background to styles["background"]
• Tabulator tests.
• Dial not having a _widget_type.

Also:

• Remove a version guard for bokeh.

---

Background

A lot of the failing test on branch-1.0 is because Bokeh 3 no longer uses model.background but instead uses model.styles["background"].

This PR fixes these failing tests by converting model.background -> model.styles["background"] and then gives a warning.

The changes made in this PR do not work in Firefox and will be fixed in bokeh/bokeh#12633.

[codecov]

Codecov Report

Merging #4140 (9a08244) into branch-1.0 (c4b17ba) will decrease coverage by 9.41%.
The diff coverage is 87.27%.

@@              Coverage Diff               @@
##           branch-1.0    #4140      +/-   ##
==============================================
- Coverage       83.57%   74.15%   -9.42%     
==============================================
  Files             223      222       -1     
  Lines           33234    33314      +80     
==============================================
- Hits            27774    24705    -3069     
- Misses           5460     8609    +3149     

Flag	Coverage Δ
ui-tests	?
unitexamples-tests	74.15% <87.27%> (-0.45%)	⬇️

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Impacted Files	Coverage Δ
panel/command/convert.py	32.35% <ø> (+0.92%)	⬆️
panel/io/jupyter_server_extension.py	0.00% <0.00%> (ø)
panel/io/state.py	61.97% <0.00%> (-7.85%)	⬇️
panel/layout/spacer.py	74.07% <0.00%> (ø)
panel/models/__init__.py	100.00% <ø> (ø)
panel/pane/image.py	79.06% <ø> (-0.47%)	⬇️
panel/template/fast/theme.py	100.00% <ø> (ø)
panel/template/material/__init__.py	100.00% <ø> (ø)
panel/tests/io/test_embed.py	99.42% <ø> (ø)
panel/tests/layout/test_tabs.py	100.00% <ø> (ø)
... and 121 more

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