Drop Python 3.7 support and refresh CI

[randomir]

In this PR we:

• drop py37 support
• update CI: always run the latest patch version of python (for each minor). Even on Windows! :smile:
• drop appveyor; run windows tests on circleci

[codecov[bot]]

Codecov Report

Merging #558 (c935946) into master (b302a1b) will increase coverage by 0.05%. The diff coverage is 100.00%.

:exclamation: Current head c935946 differs from pull request most recent head 59f5f79. Consider uploading reports for the commit 59f5f79 to get more accurate results

@@            Coverage Diff             @@
##           master     #558      +/-   ##
==========================================
+ Coverage   86.87%   86.92%   +0.05%     
==========================================
  Files          24       24              
  Lines        3528     3527       -1     
==========================================
+ Hits         3065     3066       +1     
+ Misses        463      461       -2     

Files Changed	Coverage Δ
dwave/cloud/client/base.py	90.50% <100.00%> (+0.23%)	:arrow_up:

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