mnt: fixed matplotlib tests in <=3.4

[pfebrer]

Solves #377

[codecov]

Codecov Report

Merging #378 (892fe63) into master (8184c75) will increase coverage by 0.00%.
The diff coverage is 87.50%.

@@           Coverage Diff           @@
##           master     #378   +/-   ##
=======================================
  Coverage   86.97%   86.98%           
=======================================
  Files         321      321           
  Lines       41752    41759    +7     
=======================================
+ Hits        36313    36323   +10     
+ Misses       5439     5436    -3     

Impacted Files	Coverage Δ
sisl/viz/backends/matplotlib/backend.py	57.53% <87.50%> (+2.98%)	⬆️
sisl/io/siesta/out.py	77.42% <0.00%> (+0.47%)	⬆️
sisl/io/gulp/got.py	86.45% <0.00%> (+0.64%)	⬆️

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