Try changing all of the \ to \\, or put an r in front of the string, e.g.,
if r.status_code == 500 and r'Sabre\VObject\Recur\NoInstancesException' in r.text:
# ^ that right there
In general, though, running Python 2.7 code with Python 3 (or vice versa) will almost certainly not work! They are not directly compatible. Stick with 2.7 if that's what your scripts were written for.
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