Use deep copy so that all arguments are copied by value

[orientalperil]

This can go wrong if you use a list to store validators. With shallow copy the validators list is maintained even if a new request is being processed. The result is that the validators list grows larger with every request. This is especially bad with UniqueValidator because old sessions get stuck in the list and you will get SQLAlchemy transaction errors like sqlalchemy.exc.StatementError: (sqlalchemy.exc.InvalidRequestError) Can't reconnect until invalid transaction is rolled back

[sourcery-ai[bot]]

Sourcery Code Quality Report

❌  Merging this PR will decrease code quality in the affected files by 0.01%.

Quality metrics	Before	After	Change
Complexity	8.23 ⭐	8.23 ⭐	0.00
Method Length	58.00 ⭐	58.02 ⭐	0.02 👎
Working memory	10.55 😞	10.55 😞	0.00
Quality	64.46% 🙂	64.45% 🙂	-0.01% 👎

Other metrics	Before	After	Change
Lines	784	784	0

Changed files	Quality Before	Quality After	Quality Change
rest_witchcraft/serializers.py	64.46% 🙂	64.45% 🙂	-0.01% 👎

Here are some functions in these files that still need a tune-up:

File	Function	Complexity	Length	Working Memory	Quality	Recommendation
rest_witchcraft/serializers.py	ModelSerializer.perform_update	43 ⛔	190 😞	17 ⛔	20.57% ⛔	Refactor to reduce nesting. Try splitting into smaller methods. Extract out complex expressions
rest_witchcraft/serializers.py	ModelSerializer.get_field_names	17 🙂	191 😞	13 😞	38.24% 😞	Try splitting into smaller methods. Extract out complex expressions
rest_witchcraft/serializers.py	BaseSerializer.build_standard_field_kwargs	11 🙂	140 😞	19 ⛔	41.78% 😞	Try splitting into smaller methods. Extract out complex expressions
rest_witchcraft/serializers.py	ModelSerializer.build_nested_field	6 ⭐	145 😞	14 😞	50.91% 🙂	Try splitting into smaller methods. Extract out complex expressions
rest_witchcraft/serializers.py	ExpandableModelSerializer.to_representation	4 ⭐	91 🙂	22 ⛔	52.92% 🙂	Extract out complex expressions

Legend and Explanation

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• ⭐ excellent
• 🙂 good
• 😞 poor
• ⛔ very poor

The 👍 and 👎 indicate whether the quality has improved or gotten worse with this pull request.

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