Ensure significand remains an integer in Python3 json parser

Ensure significand remains an integer in Python3 json parser

The / operation in Python 2 is "floor division" for int/long types
while in Python 3 is "true division". This means that the
significand can become a float with the existing code in Python 3.
This, in turn, can result in a parse of something like [1.10e1]
returning 11 in Python 2 and 11.0 in Python 3. Switching to the
// operator resolves this difference.

The JSON tests do not catch this difference because the built-in
serializer prints floats with the %.15g format which will convert
floats with no fractional part to an integer representation.

Signed-off-by: Terry Wilson <twilson@redhat.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>