Enforce hash keys on 2.1 external-references

[chisholm]

Fixes #359

[codecov-io]

Codecov Report

Merging #360 into master will decrease coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #360      +/-   ##
==========================================
- Coverage   98.16%   98.14%   -0.02%     
==========================================
  Files         124      124              
  Lines       14158    13918     -240     
==========================================
- Hits        13898    13660     -238     
+ Misses        260      258       -2

Impacted Files	Coverage Δ
stix2/test/v21/test_external_reference.py	100% <100%> (ø)	⬆️
stix2/v21/common.py	100% <100%> (ø)	⬆️
stix2/utils.py	97.15% <0%> (-0.57%)	⬇️
stix2/patterns.py	98% <0%> (-0.02%)	⬇️
stix2/test/v20/test_datastore_filesystem.py	99.19% <0%> (-0.02%)	⬇️
stix2/test/v21/test_relationship.py	100% <0%> (ø)	⬆️
stix2/test/v20/test_campaign.py	100% <0%> (ø)	⬆️
stix2/test/v21/test_core.py	100% <0%> (ø)	⬆️
stix2/test/v21/test_attack_pattern.py	100% <0%> (ø)	⬆️
stix2/test/v21/test_custom.py	100% <0%> (ø)	⬆️
... and 61 more

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[clenk]

LGTM!