Sorry, I had this floating on my mind and couldn't wait anymore to publish it :sweat_smile:. This is just a WIP for now, so you can have an example implementation, and if it gets approved, I can start working with tests and docs.

In order to make possible to implement a different workflow on downstream addon maintenance teams, as explained in https://github.com/OCA/maintainer-tools/issues/334, Odoo needs to support version-specific code.

This can be currently done in Python by checking `odoo.release.version_info` and in JS by using the `/web/webclient/version_info` controller. However, when dealing with data files, we were out of luck.

With this patch:

- A new cached helper is found in JS sessions, that lazy-caches version info once per session, which will make version-specific assertions a pleasure in JS.
- A new attribute `versions` is supported in `<odoo>`, `<openerp>` and `<data>` elements within XML data files, that allows the programmer to specify a version spec that makes that data section be omitted when Odoo version doesn't match it.
- A new `overrides` key is supported in the `__manifest__.py` file, which has the form of a dict, where keys are version specs, and values are dicts that will directly override the addon base manifest.

So, for instance, to declare an addon to be available only until version 13.0 of Odoo, put this in the manifest:

```python
{
  ...
  "installable": False,
  "overrides": {
    ":13": {
      "installable": True,
    }
  }
}
```


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