PEP 660: Final touches in main new fragment

Author: abravalheri

changelog.d/3265.breaking.rst

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+Added implementation for *editable install* hooks (PEP 660).
+
+By default the users will experience a *lenient* behavior  which prioritises
+the ability of the users of changing the distributed packages (e.g. adding new
+files or removing old ones).
+But they can also opt into a *strict* mode, which will try to replicate as much
+as possible the behavior of the package as if it would be normally installed by
+end users. The *strict* editable installation is not able to detect if files
+are added or removed from the project (a new installation is required).
+
+.. important::
+   The *editable* aspect of the *editable install* supported this implementation
+   is restricted to the Python modules contained in the distributed package.
+   Changes in binary extensions (e.g. C/C++), entry-point definitions,
+   dependencies, metadata, datafiles, etc may require a new installation.