Rename Scatter mark to Dots

[mwaskom]

Scatter was the first mark added for prototyping before much though about general naming schemes. We've since settled on a pattern where a plural form of a mark indicates that the mark is better suited to plots where it will appear many times. This scheme applies to the Dot/Scatter relationship too, so best to be consistent. Dots is also almost half as long as Scatter. It's also good in that Scatter did not produce plots that looked particularly like the existing scatterplot.

The scatter.py module is now accordingly changed to dot.py.

As part of the change, refactored the relationship between Dot and Dots so that they depend on a common base rather than having one inherit from the other.

[codecov]

Codecov Report

Merging #2942 (b9f185f) into master (9771eae) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #2942   +/-   ##
=======================================
  Coverage   98.30%   98.30%           
=======================================
  Files          69       69           
  Lines       22985    22996   +11     
=======================================
+ Hits        22596    22607   +11     
  Misses        389      389           

Impacted Files	Coverage Δ
seaborn/_core/properties.py	99.15% <ø> (ø)
seaborn/_marks/dot.py	100.00% <100.00%> (ø)
seaborn/objects.py	100.00% <100.00%> (ø)
tests/_marks/test_dot.py	100.00% <100.00%> (ø)