Use our `python_version.sh` script everywhere
I noticed that we were reading our `.python-version` file directly,
rather than using the `python_version.sh` script that we use
elsewhere. Even though the results will be the same, it's good to use
the same code everywhere.
IMAGE_TAG ?= dev
# This is a build argument for our Python images, and is thus used in
# docker-bake.hcl
-PYTHON_VERSION ?= $(shell cat .python-version)
+PYTHON_VERSION ?= $(shell build-support/python_version.sh)
# This is a build argument for our Rust images, and is thus used in
# docker-bake.hcl
RUST_VERSION ?= $(shell build-support/rust_version.sh)
