Add required auto_created flag to TaxedMoneyField, fix code style and bump ver. to Beta 5

[rafalp]

Before releasing Beta 4 I've cut the auto_created flag from TaxedMoneyField based on what I've looked up on Django source and our test suite. This passed the test, but apparently the flag was required by Django's query building facilities for resolving possible reverse relations.

Code from this PR was tested with local Saleor fork that I'm using to develop price handling update. All tests are now passing with model using this field successfully saving to DB and then deleting from it. I've also tested makemigrations for it and found out that its ignored and its creation on model doesn't cause new migration to appear, which imho is what we want.

[codecov[bot]]

Codecov Report

Merging #56 into master will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #56   +/-   ##
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  Coverage   96.15%   96.15%           
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  Files           4        4           
  Lines         104      104           
  Branches       10       10           
=======================================
  Hits          100      100           
  Misses          2        2           
  Partials        2        2

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[patrys]

Was beta4 ever released?

[rafalp]

Yup, 4 hours ago: https://github.com/mirumee/django-prices/releases/tag/1.0.0-beta4