Prepare for Python3

[eumiro]

Two independent commits:

• from __future__ import division and then apply it to two integers where a / b returns a float
• simplify multiple isinstance calls

[nicolaiarocci]

thanks!

[nicolaiarocci]

@eumiro with the advent of pymongo 4 (also see #1461) which only supports Python 3, it might be the right time for the switch. Would you be interested in tackling the issue?