pp.highly_variable_genes handles subset and inplace consistently

[eroell]

• Fixes sc.pp.highly_variable does not always handle subset and inplace arguments consistently #1867
• Tests included or not required because: New tests included which catch the failure mode described in sc.pp.highly_variable does not always handle subset and inplace arguments consistently #1867. Current implementation fails these.

• Release notes not necessary because: Added entry in release notes.

Addresses issue #1867 with a fix as outlined by @jlause and tests which catch the failure mode detected and nicely demonstrated by @jlause.

[codecov]

Codecov Report

Merging #2757 (ac419a2) into master (bc349b9) will increase coverage by 0.00%.
The diff coverage is 83.33%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2757   +/-   ##
=======================================
  Coverage   72.46%   72.47%           
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  Files         111      111           
  Lines       12418    12422    +4     
=======================================
+ Hits         8999     9003    +4     
  Misses       3419     3419           

Files	Coverage Δ
scanpy/preprocessing/_highly_variable_genes.py	96.03% <83.33%> (+0.07%)	⬆️