Use setuptools_scm to simplify versioning during release

[hugovk]

No editing of files, just add the version tag.

Can check with:

$ python setup.py --version
0.7.3.dev85

Also add Source metadata to project_urls, which can be useful on PyPI and for programmatic access to packages.

[codecov]

Codecov Report

Merging #34 into master will increase coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master      #34      +/-   ##
==========================================
+ Coverage   83.25%   83.26%   +0.01%     
==========================================
  Files           2        2              
  Lines        1427     1428       +1     
==========================================
+ Hits         1188     1189       +1     
  Misses        239      239              

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