MNT: add _version.py to .gitignore

[StefRe]

PR summary

_version.py is generated by setuptools_scm when you do an editable install and should not be tracked in version control.

PR checklist

• [N/A] "closes #0000" is in the body of the PR description to link the related issue
• [N/A] new and changed code is tested
• [N/A] Plotting related features are demonstrated in an example
• [N/A] New Features and API Changes are noted with a directive and release note
• [N/A] Documentation complies with general and docstring guidelines

[oscargus]

It should definitely be ignored. Strange that it wasn't earlier...

(Although I most often use git commit -a ... so that may be why I haven't noticed it.)

[QuLogic]

If you have a _version.py, it's a leftover from some other branch or older commit. The Meson build puts _version.py in the build directory, not the source. This was removed on purpose, as you should be deleting it if you have the leftover file.

[StefRe]

Yes, my bad: the _version.py in lib\matplotlib\ was created by a v3.8 install, v3.9 creates the file in build\cp311\lib\matplotlib\ which is ignored.