Update shebangs to comply with PEP 394

Change the python scripts to call python instead of python3, as this
works cross platform. See:
https://peps.python.org/pep-0394/#for-python-script-publishers

hw/boards/mta1-usb-v1/test/connected_test.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python3
+#!/usr/bin/env python
 import hid_test
 
 def import_pins(pin_filename):

hw/boards/mta1-usb-v1/test/connected_test_gui.py

@@ -1,4 +1,4 @@
-#!/usr/bin/python3
+#!/usr/bin/python
 import tkinter as tk
 from tkinter import filedialog as fd
 from tkinter.messagebox import askyesno

hw/boards/mta1-usb-v1/test/encode_usb_strings.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python3
+#!/usr/bin/env python
 
 manufacturer = 'Mullvad'
 product = 'MTA1-USB-V1'

hw/boards/mta1-usb-v1/test/production_test.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python3
+#!/usr/bin/env python
 import hid_test
 import time
 import numpy

hw/boards/mta1-usb-v1/test/reset.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python3
+#!/usr/bin/env python
 import hid_test
 import time