Drop python 2 support

[rwedge]

This PR removes Python 2 support from Featuretools

After creating the pull request: in order to pass the changelog_updated check you will need to update the "Future Release" section of docs/source/changelog.rst to include this pull request.

[codecov]

Codecov Report

Merging #759 into master will increase coverage by 0.21%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #759      +/-   ##
==========================================
+ Coverage   97.76%   97.97%   +0.21%     
==========================================
  Files         119      119              
  Lines       10861    10772      -89     
==========================================
- Hits        10618    10554      -64     
+ Misses        243      218      -25

Impacted Files	Coverage Δ
featuretools/computational_backends/feature_set.py	100% <ø> (ø)	⬆️
featuretools/feature_base/feature_base.py	97.88% <ø> (-0.01%)	⬇️
featuretools/utils/cli_utils.py	55.31% <ø> (ø)	⬆️
featuretools/wrappers/sklearn.py	95.45% <ø> (-0.11%)	⬇️
featuretools/utils/entity_utils.py	98.03% <ø> (-0.02%)	⬇️
...aturetools/primitives/standard/binary_transform.py	100% <ø> (ø)	⬆️
...eaturetools/tests/utils_tests/test_entity_utils.py	100% <ø> (ø)	⬆️
...eaturetools/tests/entityset_tests/test_plotting.py	100% <ø> (ø)	⬆️
...computational_backends/calculate_feature_matrix.py	98.22% <ø> (-0.02%)	⬇️
...ols/tests/synthesis/test_deep_feature_synthesis.py	99.06% <ø> (ø)	⬆️
... and 37 more

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[frances-h]

LGTM 👍