Update CompositeDataSource and deduplicate() support for unversioned objects

[chisholm]

Fixes #441 .

Update CompositeDataSource and deduplicate() to handle unversioned objects, including 2.1 SCOs.

[clenk]

Travis is failing: https://travis-ci.org/github/oasis-open/cti-python-stix2/jobs/719321125

Not sure why it's not showing up here on GitHub anymore.

[chisholm]

I'm pretty sure the pattern_visitor.py and test_pattern_expression.py stuff is fixed in the pattern equivalence PR. If I fixed it here, there would be merge conflicts there. I fixed the missing comma in conftest.py though.

[maybe-sybr]

#449 looks pretty big - is the expectation that it's good to go in now that it's up or would it be better to pull the lint fixes out and get master stable separately so this and other PRs can go in?

[emmanvg]

@maybe-sybr I will rebase from master once the pre-commit fixes make it. We definitely want to make sure we are covering new code and tests.

[codecov-commenter]

Codecov Report

Merging #444 into master will decrease coverage by 0.14%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #444      +/-   ##
==========================================
- Coverage   98.39%   98.25%   -0.15%     
==========================================
  Files         131      142      +11     
  Lines       14942    15728     +786     
==========================================
+ Hits        14702    15453     +751     
- Misses        240      275      +35     

Impacted Files	Coverage Δ
stix2/datastore/__init__.py	100.00% <100.00%> (ø)
stix2/pattern_visitor.py	93.30% <100.00%> (+1.80%)	⬆️
stix2/patterns.py	99.35% <100.00%> (+<0.01%)	⬆️
stix2/test/v20/test_pattern_expressions.py	100.00% <100.00%> (ø)
stix2/test/v20/test_utils.py	100.00% <100.00%> (ø)
stix2/test/v21/conftest.py	100.00% <100.00%> (ø)
stix2/test/v21/test_datastore_composite.py	100.00% <100.00%> (ø)
stix2/test/v21/test_pattern_expressions.py	100.00% <100.00%> (ø)
stix2/test/v21/test_utils.py	100.00% <100.00%> (ø)
stix2/utils.py	98.33% <100.00%> (+1.66%)	⬆️
... and 13 more

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