BUG: import of maybe_convert_indices in pandas.core.index.py, #10610
closes #10610 

I fixed the import statement and added a test to check for proper behavior when accessing a mixed-integer index with a list of values.

I removed the index checking if-statement, and added a line that would change the -1 indices to equal the length of the Index being evaluated, so that the IndexError would be raised in maybe_convert_indices.
 ok, this is fine.

pls squash to a single commit. ping when green.
 merged via https://github.com/pydata/pandas/commit/12df3ea39b28f752a0dcb43b87c4121387114cdf

thanks!
