Avoid pandas deprecation warning

[martinholmer]

The current code produces this pandas deprecation warning:

     /Users/mrh/work/Tax-Calculator/taxcalc/records.py:353:
        FutureWarning:
     Series.__getitem__ treating keys as positions is deprecated.
        In a future version, integer keys will always be treated as
        labels (consistent with DataFrame behavior). To access a value
        by position, use `ser.iloc[pos]`
     self.e00300 *= self.ADJ['INT{}'.format(year)][self.agi_bin].values

This pull request makes the recommended code revision.
This revision causes no change in tax calculation output.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (d806ac3) 98.54% compared to head (3a1eefe) 98.54%.
Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2715   +/-   ##
=======================================
  Coverage   98.54%   98.54%           
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  Files          14       14           
  Lines        2616     2616           
=======================================
  Hits         2578     2578           
  Misses         38       38           

Flag	Coverage Δ
unittests	98.54% <100.00%> (ø)

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Files	Coverage Δ
taxcalc/records.py	100.00% <100.00%> (ø)

[jdebacker]

Thank you @martinholmer! Merging.