Allow asking for a key of obs when use_raw=True in deprecated _get_{obs,var}_array

[ivirshup]

Part of scverse/scanpy#728 and releasing v0.6.22

This throws an error on master:

import scanpy as sc
pbmc = sc.datasets.pbmc68k_reduced()
sc.pl.scatter(pbmc, "n_genes", "HES1")

It doesn't after this commit.

All this does is fix a backward breaking change. sc.pl.scatter still needs to be fixed so it doesn't use the deprecated function, but this is a little more challenging.

[codecov]

Codecov Report

Merging #176 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master     #176   +/-   ##
=======================================
  Coverage   78.85%   78.85%           
=======================================
  Files          12       12           
  Lines        2109     2109           
=======================================
  Hits         1663     1663           
  Misses        446      446

Impacted Files	Coverage Δ
anndata/core/anndata.py	79.38% <100%> (ø)	⬆️

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