dev: add .python-version file so pyenv isn't user/system specific (#2883)

The `.python-version` file is read by pyenv to decide what Python
version to use.

This makes it easier to get started with the project with installing
things like pre-comment
since you don't have to figure out what version of Python you need.

.python-version

diff --git a/.python-version b/.python-version
new file mode 100644
index 0000000..2c20ac9
--- /dev/null
+++ b/.python-version
@@ -0,0 +1 @@
+3.13.3