add pixi configure in python gitignore

[kasi-x]

Reasons for making this change:

pixi is a package manager.
I want to append pixi support for this project.
This appendix is almost same as previous ones, pyenv, pipenv(#2977) pdm(##3976), poetry(#3853).

This change include two type of changes for pixi's support.

First one is for pixi.lock. It just follows precedent cases(pipenv, poetry, pdm).
I don't think there is any problem to append this line.

Second one is for .pixi.
pixi create .pixi directory, just like Python's venv module creates one in the .venv directory. .pixi is a directory for setting up virtualenv.
It is recommended not to include this directory in version control in pixi system.

Why .pixi is needed in gitignore.

When initialize pixi project (by running pixi init command), pixi automatically add the line of .pixi to .gitignore file. (If already exist .pixi in user's .gitignore file, pixi doesn't change anything.)
But user may use this project's template after initialized pixi.
This change doesn't cause any breaking change. Appending .pixi in this project's template is reasonable.

• Links to documentation supporting these rule changes:
  https://prefix.dev/docs/pixi/cli#init

• Link to application or project’s homepage: https://pixi.sh/latest/