fix ordering problem with Class definitions

[emmanvg]

closes #472

[codecov-io]

Codecov Report

Merging #474 (e7242c9) into master (3cef32e) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #474   +/-   ##
=======================================
  Coverage   89.33%   89.33%           
=======================================
  Files         144      144           
  Lines       16020    16020           
=======================================
  Hits        14311    14311           
  Misses       1709     1709           

Impacted Files	Coverage Δ
stix2/test/v21/test_malware_analysis.py	100.00% <ø> (ø)
stix2/v20/common.py	49.33% <ø> (ø)
stix2/v21/common.py	44.28% <ø> (ø)
stix2/v21/observables.py	34.69% <ø> (ø)
stix2/v21/sdo.py	59.44% <ø> (ø)

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[clenk]

After looking through the examples in the spec, this looks like this lines up with the ordering used in those examples. 👍