Updated py-setuptools_scm to 1.15.0.

v1.15.0
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* more sophisticated ignoring of mercurial tag commits
  when considering distance in commits
  (thanks Petre Mierlutiu)
* fix issue #114: stop trying to be smart for the sdist
  and ensure its always correctly usign itself
* update trove classifiers
* fix issue #84: document using the installed package metadata for sphinx
* fix issue #81: fail more gracious when git/hg are missing
* address issue #93: provide an experimental api to customize behaviour on shallow git repos
  a custom parse function may pick pre parse actions to do when using git

v1.14.1
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* fix #109: when detecting a dirty git workdir
            don't consider untracked file
            (this was a regression due to #86 in v1.13.1)
* consider the distance 0 when the git node is unknown
  (happens when you haven't commited anything)

(wiz)