Odoo-test-helper

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[FIX] FakeModelLoader: avoid loading module again on restore_registry() for Odoo 15.0+
[FIX] Ensures registy is cleaned up in addons without models

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- [IMP] Allow to ignore Odoo core modules to avoid warning

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- .gitignore added

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- [FIX] AttributeError in Odoo 15+, regression introduced in 2.0.3

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- [FIX] restore_registry in Odoo 16

See odoo/odoocd12293

This new attribute is the source of truth for the base classes
and in setup_models (called further down in the modified code in this PR),
the model's base classes are reset from it:

https://github.com/odoo/odoo/blob/e1f06479a526c703ccabc441b1e194646206b966/odoo/models.py#L2728-L2730.

The test failure fixed by this PR can be inspected in
https://app.travis-ci.com/github/OCA/odoo-test-helper/builds/258453331

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- Fix ``mock`` import for v15