Stop tracking generated _version.py

[vineetbansal]

Closes #3476.

Some background on this is that backends that support auto-versioning (by looking at the state of the working tree against the current git branch+tag+commit), auto-generate a _version.py (or a similarly named file) when doing a poetry install or uv install etc, and don't need a _version.py file in the git repo (a file they're going to overwrite anyway).

Having this file in the git index causes developers to keep accidentally committing/pushing it. Its entry was added in .gitignore, but perhaps too late (after it had already been committed to the git index).

If some code relies on this file being present, it should guard against its absence with an ImportError. There are other ways to do this "default" version instead of hardcoding it, but it gets very specific to the backend, for example, this page says:

- persistent-substitution (boolean, optional): If true, then do not revert any substitutions applied to this file. This is primarily useful for editable installs, if you need the version to remain in a file ignored by your VCS.
- initial-content (string, optional): Set the file content before the substitution phase. The file will be created or overwritten as necessary. Common leading whitespace will be stripped from each line.

IMO it's easier to do the right thing and just have a default version specified on an ImportError, which is more explicit, as we're doing here.

[leowerneck]

LGTM

[lacoak21]

LGTM too!