Drop Python 3.7 and refresh CI

[randomir]

• drop py37
• upgrade xcode to fix CircleCI macos builds on the medium resource class
• always run on the latest patch version of python (includes a workaround on windows)

[codecov[bot]]

Codecov Report

Merging #159 (6bb5933) into master (a3cf610) will not change coverage. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #159   +/-   ##
=======================================
  Coverage   88.08%   88.08%           
=======================================
  Files           8        8           
  Lines         772      772           
=======================================
  Hits          680      680           
  Misses         92       92           

Files Changed	Coverage Δ
dwave/inspector/server.py	81.69% <100.00%> (ø)

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