Fix AppVeyor Breakage

[ischoegl]

Checklist

• There is a clear use-case for this code change
• The commit message has a short title & references relevant issues
• Build passes (scons build & scons test) and unit tests address code coverage
• The pull request is ready for review

If applicable, fill in the issue number this pull request is fixing

Fixes #878

Changes proposed in this pull request

• Fix deprecation in np.array, where dtype no longer supports np.integer.
• Use np.uint64 instead (i.e. in analogy to C++ size_t)

[codecov]

Codecov Report

Merging #879 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #879   +/-   ##
=======================================
  Coverage   71.56%   71.56%           
=======================================
  Files         372      372           
  Lines       44510    44510           
=======================================
  Hits        31853    31853           
  Misses      12657    12657           

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[bryanwweber]

Thanks for the quick fix!