Fixes for Koalas 1.7.0

[thehomebrewnerd]

Pull Request Description

This PR implements fixes for two issues for breaking changes that occurred with Koalas 1.7.0:

• Update variable.py to make sure datetime64[ns] is used properly in place of datetime
• Add a pytest.skip to a test that was failing due to Koalas throwing additional warnings. This is similar to the approach used in another test, but could warrant further investigation to see if the extra Koalas warning can be eliminated.

[codecov]

Codecov Report

Merging #1351 (201195c) into main (ac2168d) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #1351   +/-   ##
=======================================
  Coverage   98.58%   98.58%           
=======================================
  Files         135      135           
  Lines       14541    14543    +2     
=======================================
+ Hits        14335    14337    +2     
  Misses        206      206           

Impacted Files	Coverage Δ
featuretools/tests/synthesis/test_dfs_method.py	100.00% <100.00%> (ø)
featuretools/variable_types/variable.py	98.24% <100.00%> (ø)

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[rwedge]

Looks good!