Fix a SyntaxWarning in Python 3.8+

[vEpiphyte]

Python 3.8+ now produces a SyntaxWarning when identify checks are used with certain literals. This was documented in the 3.8 release notes here https://docs.python.org/3/whatsnew/3.8.html#porting-to-python-3-8

On import, the oletools package currently produces a such a warning message:

/usr/local/lib/python3.8/dist-packages/oletools/oleobj.py:581: SyntaxWarning: "is" with a literal. Did you mean "=="?
  if idx is -1:

This PR changes the identify check to a equality check.