python 3 support

[andrewspiers]

In [1]: from fit import FitFile
Traceback (most recent call last):

File "/mnt/c/Users/andre/Documents/FIT experiment/venv/lib/python3.6/site-packages/IPython/core/interactiveshell.py", line 3319, in run_code
exec(code_obj, self.user_global_ns, self.user_ns)

File "", line 1, in
from fit import FitFile

File "/mnt/c/Users/andre/Documents/FIT experiment/venv/lib/python3.6/site-packages/fit/init.py", line 53
raise exc_type, exc_val, exc_tb
^
SyntaxError: invalid syntax