Support STIX 2.1 version precision

[chisholm]

Fixes #350 , #329

The new rules say versioning timestamps need at least millisecond precision; the old ones said exactly millisecond precision. So we are able to allow more precision in those timestamps now. This PR adds support for expressing and enforcing a minimum precision on versioning timestamps.

[chisholm]

There is a test failure specific to Python 3.6. I think it might be a result of:

https://bugs.python.org/issue32404

My unit test uses utcnow() whereas the bug report describes fromtimestamp(), but I think both methods invoke the same underlying code which contains the bug. This is not something we can fix. All we can do is work around it for that particular unit test and version of Python.

[codecov-io]

Codecov Report

Merging #374 into master will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #374      +/-   ##
==========================================
+ Coverage   98.18%   98.19%   +0.01%     
==========================================
  Files         124      125       +1     
  Lines       14238    14333      +95     
==========================================
+ Hits        13980    14075      +95     
  Misses        258      258              

Impacted Files	Coverage Δ
stix2/test/v21/test_attack_pattern.py	100.00% <ø> (ø)
stix2/v21/common.py	100.00% <ø> (ø)
stix2/v21/sdo.py	94.44% <ø> (ø)
stix2/v21/sro.py	94.73% <ø> (ø)
stix2/properties.py	98.36% <100.00%> (+<0.01%)	⬆️
stix2/test/v21/test_timestamp_precision.py	100.00% <100.00%> (ø)
stix2/test/v21/test_versioning.py	100.00% <100.00%> (ø)
stix2/utils.py	97.64% <100.00%> (+0.46%)	⬆️

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[clenk]

LGTM; thanks @chisholm!