Upgrade script now looks for Python path as env var

upgrade.sh

@@ -2,8 +2,13 @@
 # This script will prepare NetBox to run after the code has been upgraded to
 # its most recent release.
 
+# This script will invoke Python with the value of the PYTHON environment
+# variable (if set), or fall back to "python3". Note that NetBox v3.0+ requires
+# Python 3.7 or later.
+
 cd "$(dirname "$0")"
 VIRTUALENV="$(pwd -P)/venv"
+PYTHON="${PYTHON:-python3}"
 
 # Remove the existing virtual environment (if any)
 if [ -d "$VIRTUALENV" ]; then
@@ -15,7 +20,7 @@ else
 fi
 
 # Create a new virtual environment
-COMMAND="python3 -m venv ${VIRTUALENV}"
+COMMAND="${PYTHON} -m venv ${VIRTUALENV}"
 echo "Creating a new virtual environment at ${VIRTUALENV}..."
 eval $COMMAND || {
   echo "--------------------------------------------------------------------"