Ensure Python Identifier Compliance When Generating Slugs

[Dresdn]

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Checklist

• I have double checked that there are no unrelated changes in this pull request (old patches, accidental config files, etc.)
• I have created at least one test case for the changes I have made
• [] I have updated the documentation for the changes I have made
• [] I have added my changes to the CHANGELOG.md

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• Bugfix
• Feature
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• Refactoring (no functional changes, no api changes)
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Related issue(s)

• Closes Slugs with Hyphens Cannot Be Accessed via Entity __getattr__ #636

Other Information

• This changes Attribute slugs to be "Python identifier compliant" by modifying how slugs are generated and validated. Doing this, all slugs can be referenced via dot notation now, eg providing name="my slug", the attribute can be accessed via instance.eav.my_slug

• All special characters are converted to unicode, so Attributes created with "Age #" and "Age %" won't conflict, as they would by just using Django's utils.slugify().