Pattern AST creation bug with object path numeric index steps

[chisholm]

from stix2.pattern_visitor import create_pattern_object

ast = create_pattern_object(pattern="[a:b[1]=2]")

print(ast)

results in a long stacktrace ending with:

  File "C:\Programming\python\stix\cti-python-stix2\stix2\pattern_visitor.py", line 262, in visitObjectPath
    return self.instantiate("ObjectPath", children[0].getText(), property_path)
  File "C:\Programming\python\stix\cti-python-stix2\stix2\pattern_visitor.py", line 69, in instantiate
    return klass_to_instantiate(*args)
  File "C:\Programming\python\stix\cti-python-stix2\stix2\patterns.py", line 314, in __init__
    self.property_path = [
  File "C:\Programming\python\stix\cti-python-stix2\stix2\patterns.py", line 316, in <listcomp>
    _ObjectPathComponent.create_ObjectPathComponent(x)
  File "C:\Programming\python\stix\cti-python-stix2\stix2\patterns.py", line 251, in create_ObjectPathComponent
    if component_name.endswith("_ref"):
AttributeError: 'IntegerConstant' object has no attribute 'endswith'

[clenk]

Fixed in #438.