Fix literal attribute access on Python 3.6 and 3.7

[hauntsaninja]

Fixes #50

[hauntsaninja]

Compare to the version in Python 3.6's Tools/parser/unparse.py:

if ((isinstance(t.value, ast.Num) and isinstance(t.value.n, int))
           or (isinstance(t.value, ast.Constant) and isinstance(t.value.value, int))):