Drop support for python < 3.6. Fix tests

[zzz4zzz]

• drop support for python < 3.6
• fix tests

[codecov-io]

Codecov Report

Merging #59 (0516e25) into master (293a2d0) will increase coverage by 1.33%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #59      +/-   ##
==========================================
+ Coverage   92.60%   93.93%   +1.33%     
==========================================
  Files           8        7       -1     
  Lines         257      264       +7     
  Branches       37       37              
==========================================
+ Hits          238      248      +10     
+ Misses         15       12       -3     
  Partials        4        4              

Impacted Files	Coverage Δ
sklearn_crfsuite/_fileresource.py	84.90% <ø> (-0.28%)	⬇️
sklearn_crfsuite/scorers.py	100.00% <ø> (ø)
sklearn_crfsuite/estimator.py	95.71% <100.00%> (+0.25%)	⬆️
sklearn_crfsuite/metrics.py	94.87% <100.00%> (+0.75%)	⬆️

---

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 293a2d0...0516e25. Read the comment docs.