Drop python 3.6 starts testing python 3.9

[franchuterivera]

address #1156

[codecov]

Codecov Report

Merging #1157 (f4902e1) into development (ba3f76f) will decrease coverage by 0.01%.
The diff coverage is n/a.

@@               Coverage Diff               @@
##           development    #1157      +/-   ##
===============================================
- Coverage        85.81%   85.79%   -0.02%     
===============================================
  Files              137      137              
  Lines            10625    10625              
===============================================
- Hits              9118     9116       -2     
- Misses            1507     1509       +2     

Impacted Files	Coverage Δ
...eline/components/feature_preprocessing/fast_ica.py	91.30% <0.00%> (-6.53%)	⬇️
...ature_preprocessing/select_rates_classification.py	87.32% <0.00%> (+1.40%)	⬆️

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