Address issues with new categorical scatterplots

[mwaskom]

Fixes #2835

• Fixes a simple bug introduced in the refactor (Refactoring and enhancing categorical plots #2429) where catplot(..., kind="swarm") would only have points in the final axes swarmed.
• Fixes a bug where catplot() with either scatterplot option was using the color cycle for each categorical level.
• Added a warning and some backwards compatibility for cases where palette= is passed without hue= in the refactored categorical scatterplots. I thought I had done this during the refactor, but apparently not. While we are changing the default behavior, it seems reasonable to honor this explicit ask for multiple colors with a warning / smoother deprecation.

[codecov]

Codecov Report

Merging #2845 (fb82139) into master (de1ecf0) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #2845   +/-   ##
=======================================
  Coverage   98.05%   98.05%           
=======================================
  Files          69       69           
  Lines       22187    22218   +31     
=======================================
+ Hits        21755    21786   +31     
  Misses        432      432           

Impacted Files	Coverage Δ
seaborn/_oldcore.py	98.07% <100.00%> (+<0.01%)	⬆️
seaborn/categorical.py	98.27% <100.00%> (+0.01%)	⬆️
tests/test_categorical.py	99.77% <100.00%> (+<0.01%)	⬆️
tests/test_core.py	99.68% <100.00%> (+<0.01%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update de1ecf0...fb82139. Read the comment docs.