Remove Python 3.15 pin on CI

[jtpio]

Added in #7836 to fix the CI:

notebook/.github/workflows/build.yml

Lines 113 to 115 in 0dd773a

	# TODO: Remove this pin once rpds-py supports Python 3.15.
	# Upstream: https://github.com/crate-py/rpds/issues/210
	python_version: '3.14'

[tsharma-dev]

Looks like rpds-py still doesn’t support Python 3.15 yet (crate-py/rpds#210 is open and blocked by pyo3).
Keeping the pin makes sense for now.