Add python-version file

Python 3.12 doesn't work for building node-sass due to the removal
of distutils, but 3.11 does. For people using pyenv (similar to nvm)
we can set a python version. This won't solve build mismatches for
everyone, but node v20 + python 3.11 does work, so might as well
capture it.

.python-version

@@ -0,0 +1 @@
+3.11