get-pip.py is downloaded even if it's not used

[edmorley]

During the Python setup step, get-pip.py is unconditionally downloaded for every build:

heroku-buildpack-python/bin/steps/python

Lines 139 to 148 in 156b07c

	# Heroku uses the get-pip utility maintained by the Python community to vendor Pip.
	# https://github.com/pypa/get-pip
	GETPIP_URL="https://lang-python.s3.amazonaws.com/etc/get-pip.py"
	GETPIP_PY="${TMPDIR:-/tmp}/get-pip.py"
	if ! curl -s "${GETPIP_URL}" -o "$GETPIP_PY" &> /dev/null; then
	mcount "failure.python.get-pip"
	echo "Failed to pull down get-pip"
	exit 1
	fi

However it's only used if performing a fresh Python install (eg new app, manually cleared cache or Python/stack version upgrade) or if the Pip version differs:

heroku-buildpack-python/bin/steps/python

Lines 150 to 159 in 156b07c

	# If a new Python has been installed or Pip isn't up to date:
	if [ "$FRESH_PYTHON" ] || [[ ! $(pip --version) == *$PIP_UPDATE* ]]; then
	puts-step "Installing pip"
	# Remove old installations.
	rm -fr /app/.heroku/python/lib/python*/site-packages/pip-*
	rm -fr /app/.heroku/python/lib/python*/site-packages/setuptools-*
	/app/.heroku/python/bin/python "$GETPIP_PY" pip=="$PIP_UPDATE" &> /dev/null

The get-pip.py download step should be inside the later conditional, so that the download can be skipped.

Doing this would save time downloading the file and remove another potential failure mode for builds.