Fix star-imports from stix2.v20 and v21#404

Merged

clenk merged 1 commit intooasis-open:masterfrom chisholm:fix_stix2_base_import

Jun 5, 2020

+61

Commits on Jun 3, 2020

Add __all__'s to v20 and v21/__init__.py to ensure that
chisholm
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