Drop Python 3.6 support

[hukkin]

TODO:

• Add changelog

sync merging with hukkin/tomli#134

[codecov-commenter]

Codecov Report

Merging #22 (9ddd49a) into master (889a15d) will not change coverage.
The diff coverage is 100.00%.

❗ Current head 9ddd49a differs from pull request most recent head 7fb9adf. Consider uploading reports for the commit 7fb9adf to get more accurate results

@@            Coverage Diff            @@
##            master       #22   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            2         2           
  Lines          118       120    +2     
  Branches        34        34           
=========================================
+ Hits           118       120    +2     

Impacted Files	Coverage Δ
tomli_w/_writer.py	100.00% <100.00%> (ø)

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