Add Boolean scale

[mwaskom]

This PR adds a new Scale object, Boolean, that has a domain of True and False.

There's a bit of a tension with boolean data: True/False can be (and often are) cast to 0/1 and treated numerically, but (even when using True and False as labels) it feels odd to sort them such that axes or legends read "False, True". Therefore, this scale aims to maintain a True, False ordering.

The scale is used by default for data with dtype bool, "boolean" (pandas' na-aware dtype), or object types (including bare Python lists) with only True/False values:

(
    so.Plot(titanic, "adult_male", "age", color="survived")
    .add(so.Dots(), so.Jitter())
)

It can also be assigned manually, which will also cast the input values to booleans:

(
    so.Plot(titanic, "adult_male", "age", color="survived")
    .add(so.Dots(), so.Jitter())
    .scale(color=so.Boolean())
)

There aren't currently many options; maybe it makes sense to add some, but for now it just accepts a single values parameter which is relevant for semantic properties (e.g., it can be a list of colors, or a palette name, or a range of point sizes, etc.).

[codecov]

Codecov Report

Merging #3205 (8ad5fed) into master (7271035) will increase coverage by 0.00%.
The diff coverage is 98.79%.

Additional details and impacted files

@@           Coverage Diff            @@
##           master    #3205    +/-   ##
========================================
  Coverage   98.42%   98.42%            
========================================
  Files          77       77            
  Lines       24179    24405   +226     
========================================
+ Hits        23798    24021   +223     
- Misses        381      384     +3     

Impacted Files	Coverage Δ
seaborn/_core/scales.py	90.65% <94.93%> (+0.56%)	⬆️
seaborn/_core/plot.py	99.33% <100.00%> (-0.01%)	⬇️
seaborn/_core/properties.py	99.52% <100.00%> (+0.33%)	⬆️
seaborn/_core/rules.py	100.00% <100.00%> (ø)
seaborn/objects.py	100.00% <100.00%> (ø)
tests/_core/test_properties.py	98.91% <100.00%> (-0.01%)	⬇️
tests/_core/test_rules.py	100.00% <100.00%> (ø)
tests/_core/test_scales.py	100.00% <100.00%> (ø)