Regenerate parser with ANTLR 4.9.2

[chisholm]

Regenerated the 2.0 and 2.1 pattern parsers with ANTLR 4.9.2. The antlr4-python3-runtime python package dependency is updated to ~=4.9.0 to agree with what ANTLR itself considers a "compatible" runtime. The parsers are also now generated with -Dlanguage=Python3 since we no longer support python2, so python2-isms which were in the generated parser code are gone now, and results in some further changes (e.g. no more u"..." style unicode string literals). I also cleaned setup.py a bit: the ANTLR runtime dependency is no longer conditional based on python version; it is redundant now that only python3+ is supported. I added a python_requires to add an overall python version dependency for the library for >=3.6, which makes more sense.

[codecov-commenter]

Codecov Report

Merging #85 (ec9c4cb) into master (e926d0a) will decrease coverage by 0.10%. The diff coverage is 90.53%.

@@            Coverage Diff             @@
##           master      #85      +/-   ##
==========================================
- Coverage   82.12%   82.02%   -0.11%     
==========================================
  Files          31       31              
  Lines        4874     4923      +49     
==========================================
+ Hits         4003     4038      +35     
- Misses        871      885      +14     

Impacted Files	Coverage Δ
stix2patterns/v20/grammars/STIXPatternVisitor.py	0.00% <0.00%> (ø)
stix2patterns/v21/grammars/STIXPatternVisitor.py	0.00% <0.00%> (ø)
stix2patterns/v21/grammars/STIXPatternParser.py	79.62% <91.36%> (+0.15%)	:arrow_up:
stix2patterns/v20/grammars/STIXPatternParser.py	80.37% <92.15%> (+0.16%)	:arrow_up:
stix2patterns/v20/grammars/STIXPatternListener.py	86.15% <98.48%> (-0.36%)	:arrow_down:
stix2patterns/v21/grammars/STIXPatternListener.py	97.01% <98.52%> (-0.68%)	:arrow_down:
stix2patterns/v20/grammars/STIXPatternLexer.py	99.67% <99.58%> (-0.33%)	:arrow_down:
stix2patterns/v21/grammars/STIXPatternLexer.py	99.67% <99.58%> (-0.33%)	:arrow_down:

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[chisholm]

The python 3.9 tests failed because it's doing pre-commit stylistic checks on generated code (the parsers). I think all style checks should be skipped for that; we should not edit it.

[clenk]

@chisholm can you add v21/grammars and v20/grammars to the exclusions in this file: https://github.com/oasis-open/cti-pattern-validator/blob/master/.pre-commit-config.yaml#L13? That should prevent the style checks on those files. We should add them as exclusions for the trailing-whitespace hook as well.

[chisholm]

Oh ok. Well, the problem wasn't that values needed to be added to the existing excludes, it was that the import sorting hook didn't have any excludes. So I added them.