Correct AMT taxable income calculation to handle QBID correctly

[martinholmer]

The AMT function in calcfunctions.py is incorrectly calculating alternative minimum taxable income because it is not subtracting the qualified business income deduction (QBID). That it should be subtracted is clear from Form 1040 and Form 6251.

Form 6251 begins this way:

.

And line 15 on Form 1040 is calculated by subtracting the QBID:

.

This subtraction of QBID is not being done in Tax-Calculator 4.2.0.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.42%. Comparing base (0b775e5) to head (57b5e18).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2784   +/-   ##
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  Coverage   99.42%   99.42%           
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  Files          13       13           
  Lines        2613     2613           
=======================================
  Hits         2598     2598           
  Misses         15       15           

Flag	Coverage Δ
unittests	99.42% <ø> (ø)

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[jdebacker]

@martinholmer Thanks for finding and fixing this bug. Merging.