BUG: position indexing in getitem with duplicated index

[jreback]

in py3, the following should correctly raise an IndexError

In [1]: s = Series(range(5),index=list('aacde'))

In [2]: s[5]
IndexError

however
a positional indexer that is in the range should work
s[3]

Further more, this must be a monotonic increasing duplicated index to trigger

In [8]: Series(range(5),index=list('aabcd'))[3]
IndexError: 3

In [9]: Series(range(5),index=list('aadcb'))[3]
Out[9]: 3

simple change needed here
in that if its an integer and in the range of the index it can return that value.

Though maybe we should just detect this in the cython code (index.pyx/Index/_get_loc_duplicated)

needless to say, no tests for this :<

[topper-123]

Removing the 2/3 compat label, as this is no longer compat related, as we now only support Python3.

[jbrockmendel]

This works on master, needs test