Code inspection improvements

[hugovk]

Plus python_requires in setup.py

[codecov]

Codecov Report

Merging #13 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master      #13   +/-   ##
=======================================
  Coverage   80.83%   80.83%           
=======================================
  Files           1        1           
  Lines         167      167           
=======================================
  Hits          135      135           
  Misses         32       32

Impacted Files	Coverage Δ
src/osmviz/manager.py	80.83% <100%> (ø)	⬆️

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