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]

Codecov Report

All modified and coverable lines are covered by tests ✅

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

Additional details and impacted files

@@            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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[zacharyburnett]

down with Python 2! Long live Python 3!

[braingram]

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