⚡ Various improvements to scatter_density

[grst]

• support for layer
• fix some type hints
• automatically infer x/y axis labels from x/y parameters
• remove arbitrary hardcoded axis limits (now defaults to max observed value in the respective dimension)

[codecov-commenter]

Codecov Report

Merging #43 (d8ad248) into main (6b37445) will increase coverage by 0.24%.
The diff coverage is 100.00%.

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@@            Coverage Diff             @@
##             main      #43      +/-   ##
==========================================
+ Coverage   80.41%   80.66%   +0.24%     
==========================================
  Files          10       10              
  Lines         388      393       +5     
==========================================
+ Hits          312      317       +5     
  Misses         76       76              

Impacted Files	Coverage Δ
pytometry/plotting/_scatter_density.py	100.00% <100.00%> (ø)

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[grst]

Python 3.9 is the lowest python version that is still supported by numpy (see "NEP29") and is also used across scverse packages.
Having Python >= 3.8 as a minimum requirement enables use of the Literal type.

[mbuttner]

OK, fine for me.