[Python] Fix indexing implementations

[asfimport]

A number of \_\_getitem\_\_ implementations handle negative or out-of-bounds indices improperly, for example:

>>> a = pa.array([11,12,13])
>>> a[-6]
11
>>> a[-15]
11
>>> a[4]
NA
>>> a[3]
NA
>>> a[1111]
NA

Reporter: Antoine Pitrou / @pitrou
Assignee: Antoine Pitrou / @pitrou

PRs and other links:

• GitHub Pull Request #1770

_{Note: This issue was originally created as ARROW-2331. Please see the migration documentation for further details.}

[asfimport]

Uwe Korn / @xhochy:
Issue resolved by pull request 1770
#1770