Upgrade default pattern to 2.1

[febrezo]

The change permits the usage of importing the v2.1 Pattern class from stix2patterns.pattern by default instead of v2.0. With this update, this:

>>> from stix2patterns.pattern import Pattern
>>> p = Pattern("[file:hashes.'SHA-256' = 'aec070645fe53ee3b3763059376134f058cc337247c978add178b6ccdfb0019f']")
>>> dir(p)
['_Pattern__do_parse', '_Pattern__parse_tree', '__class__', '__delattr__', '__dict__', '__dir__', '__doc__', '__eq__', '__format__', '__ge__', '__getattribute__', '__gt__', '__hash__', '__init__', '__init_subclass__', '__le__', '__lt__', '__module__', '__ne__', '__new__', '__reduce__', '__reduce_ex__', '__repr__', '__setattr__', '__sizeof__', '__str__', '__subclasshook__', '__weakref__', 'inspect', 'visit', 'walk']
>>> p.inspect()
pattern_data(comparisons={'file': [(['hashes', 'SHA-256'], '=', "'aec070645fe53ee3b3763059376134f058cc337247c978add178b6ccdfb0019f'")]}, observation_ops=set(), qualifiers=set())

Is now equivalent to:

>>> from stix2patterns.v21.pattern import Pattern
>>> p = Pattern("[file:hashes.'SHA-256' = 'aec070645fe53ee3b3763059376134f058cc337247c978add178b6ccdfb0019f']")
>>> dir(p)
['_Pattern__do_parse', '_Pattern__parse_tree', '__class__', '__delattr__', '__dict__', '__dir__', '__doc__', '__eq__', '__format__', '__ge__', '__getattribute__', '__gt__', '__hash__', '__init__', '__init_subclass__', '__le__', '__lt__', '__module__', '__ne__', '__new__', '__reduce__', '__reduce_ex__', '__repr__', '__setattr__', '__sizeof__', '__str__', '__subclasshook__', '__weakref__', 'inspect', 'visit', 'walk']
>>> p.inspect()
pattern_data(comparisons={'file': [(['hashes', 'SHA-256'], '=', "'aec070645fe53ee3b3763059376134f058cc337247c978add178b6ccdfb0019f'")]}, observation_ops=set(), qualifiers=set())

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[codecov-commenter]

Codecov Report

❌ Patch coverage is 0% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 82.03%. Comparing base (ead1801) to head (3d0c0cd).
⚠️ Report is 29 commits behind head on master.

Files with missing lines	Patch %	Lines
stix2patterns/pattern.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master      #95   +/-   ##
=======================================
  Coverage   82.03%   82.03%           
=======================================
  Files          31       31           
  Lines        4925     4925           
=======================================
  Hits         4040     4040           
  Misses        885      885           

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