add 2.1 links

[rpiazza]

picked a cherry!

[codecov-io]

Codecov Report

Merging #379 into master will not change coverage by %.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #379   +/-   ##
=======================================
  Coverage   98.18%   98.18%           
=======================================
  Files         124      124           
  Lines       14238    14238           
=======================================
  Hits        13980    13980           
  Misses        258      258           

Impacted Files	Coverage Δ
stix2/v21/bundle.py	100.00% <ø> (ø)
stix2/v21/common.py	100.00% <ø> (ø)
stix2/v21/observables.py	96.93% <ø> (ø)
stix2/v21/sdo.py	94.44% <ø> (ø)
stix2/v21/sro.py	94.73% <ø> (ø)

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[clenk]

LGTM, thanks @rpiazza!