Rewrite serdesertest/ to pytest

[IgorChvyrov-sm]

Changes:

Rewrote tests in the tests/serdesertest package to use pytest

Use native asserts instead of calling asserts from unittest.TestCase
Use pytest fixtures instead of unittest.TestCase setUp method
Use pytest-mock plugin instead of unittest.mock
Use pytest.raises for proper exception testing
Fixed ruff errors in the tests/serdesertest package

Reason:
Migrating from unittest to pytest for better readability, more concise test syntax and pytest powerful features.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
see 86 files with indirect coverage changes

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