Moving __future__ import to top of file

[bsb808]

Should fix the bug discovered when running the example for Dave PR: Field-Robotics-Lab/dave#149

If the future import is not at the beginning of the file, a SyntaxError is raised:

$ ./gripper_controller.py 
Traceback (most recent call last):
  File "./gripper_controller.py", line 18, in <module>
    from uuv_manipulator_interfaces import GripperInterface
  File "/home/bsb/uuv_ws/devel/lib/python3/dist-packages/uuv_manipulator_interfaces/__init__.py", line 34, in <module>
    exec(__fh.read())
  File "<string>", line 17, in <module>
  File "/home/bsb/uuv_ws/src/uuv_manipulators/uuv_manipulators_kinematics/src/uuv_manipulator_interfaces/gripper.py", line 25
    from __future__ import division
    ^
SyntaxError: from __future__ imports must occur at the beginning of the file

[M1chaelM]

This PR is a duplicate of #4 (contributed by a community member), so we should close that one after merging this. I added another small tweak to update to Python3 syntax, just in the interest of reducing the total number of PRs.

[mabelzhang]

This works for me! Thank you!

[mabelzhang]

Extra steps, but if we want to keep the earlier contribution from #4, we could merge #4, merge master into this PR, and then merge this.