fix: pin Python to 3.12

As of Ubuntu 24.04 LTS, the default version of Python is 3.12. Whereas, Ubuntu 22.04 LTS used Python 3.10, which is reaching End-of-Life next week.

Apparently, I already had created a .python-version file locally, but it was gitignored without my knowledge.

This should helps us control upgrades to Python version while some dependencies, namely `pygit2`, need extra time to add support for the new Python versions.

It also helps avoid disruption for users that are not aware of our use of a Python venv.

Author: 2bndy5

.gitignore

@@ -91,9 +91,6 @@ target/
 profile_default/
 ipython_config.py
 
-# pyenv
-.python-version
-
 # pipenv
 #   According to pypa/pipenv#598, it is recommended to include Pipfile.lock in version control.
 #   However, in case of collaboration, if having platform-specific dependencies or dependencies

.python-version

@@ -0,0 +1 @@
+3.12