remove python 2 slicing api

[braingram]

Fixes: #42

These methods are no longer supported as of python 3.

Checklist

• [ ] added entry in CHANGES.rst (either in Bug Fixes or Changes to API)
• [ ] updated relevant tests
• [ ] updated relevant documentation
• [ ] updated relevant milestone(s)
• [ ] added relevant label(s)

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 66.42%. Comparing base (4d7c3a6) to head (5bd06ec). Report is 27 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #302 +/- ## ========================================== + Coverage 64.84% 66.42% +1.58% ========================================== Files 103 101 -2 Lines 5694 5445 -249 ========================================== - Hits 3692 3617 -75 + Misses 2002 1828 -174 ```

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[braingram]

Hopefully we can remove all the 2.0 API before 4.0 is out 😜