Remove comment and set up explicit __hash__ function

[akjanik]

It turns out that in Django 2.2 bug we were unknowingly relying on was fixed. Indeed, from https://docs.python.org/3/reference/datamodel.html#object.__hash__

A class that overrides eq() and does not define hash() will have its hash() implicitly set to None.

If a class that overrides eq() needs to retain the implementation of hash() from a parent class, the interpreter must be told this explicitly by setting hash = .hash.

Additional explanation of the issue:
https://code.djangoproject.com/ticket/30333

Pull Request Checklist

1. Privileged views and APIs are guarded by proper permission checks.
2. All visible strings are translated with proper context.
3. All data-formatting is locale-aware (dates, numbers, and so on).
4. Database queries are optimized and the number of queries is constant.
5. Database migration files are up to date.
6. The changes are tested.
7. The code is documented (docstrings, project documentation).
8. GraphQL schema and type definitions are up to date.
9. Changes are mentioned in the changelog.

[codecov]

Codecov Report

Merging #3979 into master will decrease coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #3979      +/-   ##
==========================================
- Coverage   91.28%   91.27%   -0.01%     
==========================================
  Files         275      267       -8     
  Lines       14957    14514     -443     
  Branches     1450     1385      -65     
==========================================
- Hits        13653    13248     -405     
+ Misses        913      897      -16     
+ Partials      391      369      -22

Impacted Files	Coverage Δ
saleor/checkout/models.py	98.85% <100%> (-0.02%)	⬇️
saleor/account/models.py	92.03% <100%> (-0.07%)	⬇️
saleor/graphql/product/filters.py	82.85% <0%> (-7.55%)	⬇️
saleor/graphql/product/enums.py	87.87% <0%> (-2.6%)	⬇️
saleor/graphql/core/mutations.py	93.72% <0%> (-1.67%)	⬇️
saleor/product/utils/availability.py	95.45% <0%> (-1.57%)	⬇️
saleor/checkout/views/__init__.py	83.45% <0%> (-0.92%)	⬇️
saleor/graphql/product/resolvers.py	93.81% <0%> (-0.76%)	⬇️
saleor/account/forms.py	84.61% <0%> (-0.76%)	⬇️
saleor/discount/models.py	83.05% <0%> (-0.56%)	⬇️
... and 51 more

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[patrys]

Are there cases where __eq__ would return False while __hash__ remains True? Because this will break Python.