Update _dendrogram.py

[Leo-GG]

allowed the function to use specified var_names

• sc.tl.dendrogram doesn't use var_names  #1549
• Tests included or not required because:

• Release notes not necessary because:

[codecov]

Codecov Report

Merging #2765 (ceae5f2) into master (bc349b9) will not change coverage.
The diff coverage is 0.00%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2765   +/-   ##
=======================================
  Coverage   72.46%   72.46%           
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  Files         111      111           
  Lines       12418    12418           
=======================================
  Hits         8999     8999           
  Misses       3419     3419           

Files	Coverage Δ
scanpy/tools/_dendrogram.py	82.60% <0.00%> (ø)

[dburkhardt]

Hi @Leo-GG thanks for working on this! I picked this up in #2771 as I was working to generalize dendrograms to either axis.