Fix to exemption of UI from AGI for EITC in certain years

[jdebacker]

This PR addresses Issue #2660 by using a modified AGI in the calculation of the EITC.

[jdebacker]

To do:

• replicate example from Issue Incorrect 2020 EITC calculation #2660
• write a unit test of calcfunctions.EITC
• write a unit test of calcfunctions.EITCamount

[jdebacker]

Following the example in the issue pointing out this errror:

Comparing the results of two similar filing units shows the problem.

Consider this first unit:

% cat UInone.csv
RECID,MARS,e00200,e00200p,EIC,e02300
   1,   4, 19330,  19330,  3,     0
and this very similar second unit:

and this very similar second unit:

% cat UIsome.csv
RECID,MARS,e00200,e00200p,EIC,e02300
    1,   4, 19330,  19330,  3, 10200

And then using the Tax-Calculator package created from this branch, I get:

% tc UInone.csv 2020 --dump
--> UInone-20-#-#-#.csv with:
c00100 19330.0
eitc 6660.0
taxbc 68.0

and

% tc UIsome.csv 2020 --dump
--> UIsome-20-#-#-#.csv with:
c00100 29530.0
eitc 6660.0
taxbc 68.0

This gives an EITC calculation that matches TAXSIM35. Note that the AGI amount differs between TC and TAXSIM35, but the IRS guidance suggests that unemployment compensation of excluded from Taxable Income and modified AGI (used for example to compute the EITC amount), but does not affect the filer's AGI.

[codecov]

Codecov Report

Merging #2675 (6b2c8e9) into master (049e37e) will not change coverage.
The diff coverage is n/a.

Additional details and impacted files

@@           Coverage Diff           @@
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  Coverage   98.54%   98.54%           
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  Files          14       14           
  Lines        2609     2609           
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  Hits         2571     2571           
  Misses         38       38           

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[jdebacker]

I found this IRS document, which provides more clarity on how to report the exclusion. The exclusion of UI benefits will be entered as a negative in the "other income" line of Schedule 1 and therefore will affect AGI. This is what @martinholmer had suggested in Issue #2648, but it took me a while to track down the specific instructions.

This PR will be updated to reflect this.

[jdebacker]

With the latest commits (and using the CSV files as specified above), the Tax-Calculator package created from this branch returns:

% tc UInone.csv 2020 --dump
--> UInone-20-#-#-#.csv with:
c00100 19330.0
eitc 6660.0
taxbc 68.0

and

% tc UIsome.csv 2020 --dump
--> UIsome-20-#-#-#.csv with:
c00100 19330.0
eitc 6660.0
taxbc 68.0

[jdebacker]

@MattHJensen This PR is ready for your review.

[MattHJensen]

LGTM. Nice fix @jdebacker and thanks for the issue report @martinholmer.