Make S3 libraries optional

[rwedge]

This PR make smart_open, boto3, and s3fs optional dependencies, similar to graphviz.

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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-io]

Codecov Report

Merging #827 into master will increase coverage by 0.02%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #827      +/-   ##
==========================================
+ Coverage   98.12%   98.15%   +0.02%     
==========================================
  Files         119      121       +2     
  Lines       10897    10920      +23     
==========================================
+ Hits        10693    10718      +25     
+ Misses        204      202       -2

Impacted Files	Coverage Δ
featuretools/entityset/entityset.py	96.48% <100%> (+0.52%)	⬆️
featuretools/feature_base/features_serializer.py	98.07% <100%> (+0.03%)	⬆️
featuretools/feature_base/features_deserializer.py	100% <100%> (ø)	⬆️
featuretools/tests/utils_tests/test_gen_utils.py	100% <100%> (ø)
featuretools/entityset/deserialize.py	100% <100%> (ø)	⬆️
featuretools/utils/s3_utils.py	100% <100%> (ø)
featuretools/entityset/serialize.py	98.66% <100%> (+0.01%)	⬆️
featuretools/utils/gen_utils.py	91.07% <100%> (-2.91%)	⬇️

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[kmax12]

minor comments, otherwise looking good

[kmax12]

maybe also add a test that it correct does the import too?

[kmax12]

LGTM