uproot.pandas.iterate not working in python 2 (since #332)

[stderr-enst]

Looks like the public attributes start & stop of RangeIndex were only added in pandas 0.25 which is not available for python 2.
The corresponding change in uproot was introduced in #332 .
I have a setup with pandas 0.24.2 where uproot.pandas.iterate throws:

AttributeError: 'RangeIndex' object has no attribute 'start'

because of this. Using _start and _stop is probably an easy fix?

Out of curiosity: whats the general consensus, is uproot expected to support python 2 after the its phase out in 3 months? (I should probably just migrate :))

[jpivarski]

Darn—I thought that solution would work for both the old and new Pandas. I'll put in a hasattr check to cover both.

In principle, we'll still support Python 2, but we won't be testing it if the tools stop making that easy. If, for instance, conda doesn't let up install Python 2 anyone, we'll have to take it out of continuous integration. (This is the same situation we're in for Python 2.6. I still write 2.6-compliant idioms, though we don't have an easy way to test the code against 2.6.)

[jpivarski]

Let me know if PR #351 works for you. I plan to include it in uproot version 3.10.2.