feat: allow stack url overrides using BUILDPACK_VENDOR_URL env var (#643)

This is useful if you'd like to customize python binaries without forking the entire buildpack.

Similar functionality was implemented in https://github.com/heroku/heroku-buildpack-ruby/pull/238

bin/steps/python

@@ -4,6 +4,9 @@ set +e
 runtime-fixer runtime.txt
 PYTHON_VERSION=$(cat runtime.txt)
 
+# The location of the pre-compiled python binary.
+VENDORED_PYTHON="${VENDOR_URL}/runtimes/$PYTHON_VERSION.tar.gz"
+
 if [[ $PYTHON_VERSION =~ ^python-2 ]]; then
   if [[ "$PYTHON_VERSION" != "$LATEST_2" ]]; then
     puts-warn "The latest version of Python 2 is $LATEST_2 (you are using $PYTHON_VERSION, which is unsupported)."
@@ -41,7 +44,7 @@ if [ ! "$SKIP_INSTALL" ]; then
 
     mcount "version.python.$PYTHON_VERSION"
 
-    if ! curl "https://lang-python.s3.amazonaws.com/$STACK/runtimes/$PYTHON_VERSION.tar.gz" -s | tar zxv -C .heroku/python &> /dev/null; then
+    if ! curl "${VENDORED_PYTHON}" -s | tar zxv -C .heroku/python &> /dev/null; then
       puts-warn "Requested runtime ($PYTHON_VERSION) is not available for this stack ($STACK)."
       puts-warn "Aborting.  More info: https://devcenter.heroku.com/articles/python-support"
       exit 1