Use python3 in shebang

(grafted from cc8ffd818eb743822ca9014abe4f6cc2f1af5596)

--HG--
branch : 6.0

.gitlab-scripts/generate-ci.py

@@ -1,4 +1,4 @@
-#!/bin/env python
+#!/bin/env python3
 import glob
 import os
 import sys

.hooks/link_modules

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 # This file is part of Tryton.  The COPYRIGHT file at the top level of
 # this repository contains the full copyright notices and license terms.
 

.hooks/update_requirements

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 # This file is part of Tryton.  The COPYRIGHT file at the top level of
 # this repository contains the full copyright notices and license terms.
 

modules/country/scripts/import_postal_codes.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 # This file is part of Tryton.  The COPYRIGHT file at the top level of
 # this repository contains the full copyright notices and license terms.
 from __future__ import print_function

modules/purchase_shipment_cost/tests/test_purchase_shipment_cost.py

@@ -1,4 +1,3 @@
-#!/usr/bin/env python
 # This file is part of Tryton.  The COPYRIGHT file at the top level of
 # this repository contains the full copyright notices and license terms.
 import unittest