Update supported Python versions

[johannfaouzi]

Add support for Python 3.9 and drop support for Python 3.6.

[codecov]

Codecov Report

Merging #116 (6440c32) into main (61515d3) will increase coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main     #116      +/-   ##
==========================================
+ Coverage   99.04%   99.06%   +0.02%     
==========================================
  Files          97       97              
  Lines        5453     5696     +243     
==========================================
+ Hits         5401     5643     +242     
- Misses         52       53       +1     

Impacted Files	Coverage Δ
pyts/utils/deprecation.py	82.97% <0.00%> (-1.47%)	⬇️
pyts/image/gaf.py	100.00% <0.00%> (ø)
pyts/image/mtf.py	100.00% <0.00%> (ø)
pyts/metrics/dtw.py	100.00% <0.00%> (ø)
pyts/utils/utils.py	100.00% <0.00%> (ø)
pyts/tests/test_base.py	100.00% <0.00%> (ø)
pyts/approximation/mcb.py	100.00% <0.00%> (ø)
pyts/approximation/paa.py	100.00% <0.00%> (ø)
pyts/decomposition/ssa.py	100.00% <0.00%> (ø)
pyts/classification/tsbf.py	100.00% <0.00%> (ø)
... and 47 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 61515d3...6440c32. Read the comment docs.