Fix legacy 10x loader when more than one genome exists

[sjfleming]

Fixes #2246

A very small change with a small test. The new test fails for the current master branch, but passes with the fix.

[codecov[bot]]

Codecov Report

Merging #2248 (7193fd0) into master (bd06cc3) will not change coverage. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #2248   +/-   ##
=======================================
  Coverage   71.82%   71.82%           
=======================================
  Files          98       98           
  Lines       11539    11539           
=======================================
  Hits         8288     8288           
  Misses       3251     3251           

Impacted Files	Coverage Δ
scanpy/readwrite.py	67.66% <100.00%> (ø)

[sjfleming]

@ivirshup @flying-sheep I wonder if this seems like the right fix to you?

[HugoCornu]

Cool ! in order to load legacy h5, I had to freeze scanpy==1.8.2 now I included this fix in a scanpy fork

I hope it gets merged soon

[gokceneraslan]

@meeseeksdev backport to 1.9.x