Fix byte to integer conversion

[thalesmello]

Code was raising exception when:

Python 3.7.2 (default, Feb 12 2019, 08:16:38)
[Clang 10.0.0 (clang-1000.11.45.5)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import erlang
>>> from base64 import b64decode
>>> print(erlang.binary_to_term(b64decode('g3cEbmFtZQ==')))
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/thales/projects/erlang_py/erlang.py", line 421, in binary_to_term
    i, term = _binary_to_term(1, data)
  File "/Users/thales/projects/erlang_py/erlang.py", line 609, in _binary_to_term
    data[i:i + j], encoding='utf-8', errors='strict'
TypeError: unsupported operand type(s) for +: 'int' and 'bytes'

[okeuday]

Thanks for the fix! The same problem exists in a few other places and I will add a commit for those after the merge.