Star import of v20 shadows stix2.base

[maybe-sybr]

The star import from v20 in stix2/__init__.py causes the stix2.v20.base module to be aliased at stix2.base, which shadows the actual stix2/base.py module meaning it can't be accessed from external code (and is probably order of execution dependent from internal imports). This makes it hard to access the STIXJSONEncoder class from my code which has been a bit of a pain. For the moment, I just comment out the star import since I'm using explicit imports from v21 anyway.