this fixes an issue when the install script is called outside its directory

- Also reimplements the python-path finding logic of the older install.sh script.

installer/install.sh.in

@@ -3,5 +3,28 @@
 # make sure we are not already in a venv
 # (don't need to check status)
 deactivate >/dev/null 2>&1
+scriptdir=$(dirname "$0")
+cd $scriptdir
 
-exec python3 $(dirname $0)/main.py ${@}
+function version { echo "$@" | awk -F. '{ printf("%d%03d%03d%03d\n", $1,$2,$3,$4); }'; }
+
+MINIMUM_PYTHON_VERSION=3.9.0
+PYTHON=""
+for candidate in python3.10 python3.9 python3 python python3.11 ; do
+    if ppath=`which $candidate`; then
+        python_version=$($ppath -V | awk '{ print $2 }')
+        if [ $(version $python_version) -ge $(version "$MINIMUM_PYTHON_VERSION") ]; then
+            PYTHON=$ppath
+            break
+        fi
+    fi
+done
+
+if [ -z "$PYTHON" ]; then
+    echo "A suitable Python interpreter could not be found"
+    echo "Please install Python 3.9 or higher before running this script. See instructions at $INSTRUCTIONS for help."
+    read -p "Press any key to exit"
+    exit -1
+fi
+
+exec $PYTHON ./main.py ${@}

installer/installer.py

@@ -38,7 +38,8 @@ class Installer:
         self.reqs = INSTALLER_REQS
         self.preflight()
         if os.getenv("VIRTUAL_ENV") is not None:
-            raise NotImplementedError("A virtual environment is already activated. Please 'deactivate' before installation.")
+            print("A virtual environment is already activated. Please 'deactivate' before installation.")
+            sys.exit(-1)
         self.bootstrap()
 
     def preflight(self) -> None: