Added multiple database support.

[baudhuina]

This pull request is the result of my investigation discussion #325 . It revealed several issues when using multiple database and extensive tests have been added to detect them, and check they are properly fixed.

Changes:

• Now include database alias in cache key to avoid cache overlap when using multiple databases.
• Now properly support multiple databases and translatable model duplication. See multiple_db.rst for details and test_multiple_db.py for information about the tests that used to fail with v2.3.
• Now raise exceptions when attempting a call to save() or delete() which would result in an existing model to be overwritten in the target database, since overwriting the master model without carefully processing the translation can results in data corruption (see multiple_db.rst for details).
• Updated documentation about all changes, and included an overview of how save() now manages model creation, update, duplication and overwriting, in every single- and multi-database configuration (see multiple_db.rst).
• Upgraded versions in docs/_ext/djangodummy/requirements.txt to use up-to-date version of Sphinx and django. Fixed all deprecation warnings.

Backward compatibility: complete.

• All existing tests are unchanged to ensure backward compatibility, except for 2 minor mandatory changes to accommodate the new signature of get_translation_cache_key().
• There is no change in the original code of TranslatableModelMixin.save(), TranslatableModelMixin.save_translations() and translation.save() method except for the detection of the special (and rare) situations demanding either to raise an exception or to proceed differently. There is consequently no performance impact on the most common and performance-critical operations.

Test:

• Added a bunch of tests to first identify the issues with version 2.3 and cover all enhancements.
• Upgraded tox.ini and tests.yaml to cover python 3.10 and django up to 4.2 and to use coverage v7.3.0
• .coveragerc modified: now excludes tests and example.
• I couldn't run tox locally without a change in setup.py to avoid the exception on the failed import of django on line 14. This change was NOT committed, since I have to assume I missed something in my local configuration, but I couldn't figure out was: by the time setup.py is imported, django is not installed in the venv yet by tox , and the failure seems unavoidable to me. Tests are nevertheless OK without this change when run by the CI Workflow.
• With the above change, Tox was run locally (but python 3.6/3.7 tests were skipped, since those version cannot easily be installed on a Mac with M1 chip) and all tests succeed in the CI Workflow

Version and date

Version and date left open as 2.x in:

• CHANGE.rst
• multiple_db.rst
• cache.rst
• cache_design.rst
• conf.py