Add dtype to interesting_values

[frances-h]

• Add dtype to interesting_values to silence Pandas DeprecationWarning

Fixes #928

[codecov]

Codecov Report

Merging #933 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #933   +/-   ##
=======================================
  Coverage   98.17%   98.17%           
=======================================
  Files         119      119           
  Lines       10850    10861   +11     
=======================================
+ Hits        10652    10663   +11     
  Misses        198      198           

Impacted Files	Coverage Δ
featuretools/entityset/entity.py	95.68% <100.00%> (ø)
featuretools/tests/entityset_tests/test_entity.py	100.00% <100.00%> (ø)
featuretools/variable_types/variable.py	98.30% <100.00%> (+0.05%)	⬆️

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[rwedge]

Looks good

[rwedge]

Looks better