dropped legacy python support

[bmcfee]

Fixes #86 - our minimum python version is now 3.6.

[codecov-commenter]

Codecov Report

Merging #89 (780c340) into main (d064cbe) will decrease coverage by 0.31%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main      #89      +/-   ##
==========================================
- Coverage   83.80%   83.49%   -0.32%     
==========================================
  Files           4        4              
  Lines         105      103       -2     
==========================================
- Hits           88       86       -2     
  Misses         17       17              

Flag	Coverage Δ
unittests	83.49% <100.00%> (-0.32%)	⬇️

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Impacted Files	Coverage Δ
resampy/core.py	84.48% <ø> (-0.27%)	⬇️
resampy/filters.py	78.94% <100.00%> (-0.54%)	⬇️

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