Use JSONField for BasePayment.attrs

[WhyNotHugo]

Need to wait until Django 2.2 is EOL.

[mariofix]

Hi! I'm actually wanting to use this, since 2.2 has already been dropped, can I give you a hand with this?

[WhyNotHugo]

I've rebased this, but it would be good to have some more testing before merging it.

[mariofix]

I'll start using it right away, and let you know, my system processes a couple thousands purchases a day, so it's a fair trial.

[WhyNotHugo]

Tests seem to still be failing on a bunch on combinations of Django + Python, so I'd hold testing it on a production environment for a bit.

[codecov]

Codecov Report

Attention: 13 lines in your changes are missing coverage. Please review.

Comparison is base (84b52c9) 78.42% compared to head (2574ae3) 77.95%.

Files	Patch %	Lines
payments/cybersource/__init__.py	44.44%	3 Missing and 2 partials ⚠️
payments/models.py	50.00%	4 Missing ⚠️
payments/stripe/__init__.py	0.00%	3 Missing ⚠️
payments/paypal/__init__.py	88.88%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #306      +/-   ##
==========================================
- Coverage   78.42%   77.95%   -0.48%     
==========================================
  Files          29       29              
  Lines        1979     1964      -15     
  Branches      244      244              
==========================================
- Hits         1552     1531      -21     
- Misses        310      317       +7     
+ Partials      117      116       -1     

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[WhyNotHugo]

Looks like f4dfa5e is breaking some tests, but it's not clear to me why.

[WhyNotHugo]

Split the first commit into #385