Drop 3.7 and 3.8

[tony]

See also: vcs-python/libvcs#308

[codecov]

Codecov Report

Merging #356 (4b68818) into master (bd03d31) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #356   +/-   ##
=======================================
  Coverage   86.17%   86.17%           
=======================================
  Files          12       12           
  Lines         651      651           
=======================================
  Hits          561      561           
  Misses         90       90           

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