Modify fractions.gcd() import in lcm due to deprecation in Python 3.5

According to the documentation of Python 3.5, the function fractions.gcd()
is deprecated and should be replaced by math.gcd().

So we modify the import of gcd() in lcm.py to import it from the math
module when possible, keeping the import from the fractions module if not
possible in order to keep the compatibility with older versions of Python.