Add empty extras test to modules without needs

Since version 4.36.0 tox is validating extras option.
(grafted from dd354c982cd40d56844b14b553299cfb49c7c130)

--HG--
branch : 6.0

modules/product_attribute/setup.py

@@ -46,6 +46,8 @@ for dep in info.get('depends', []):
         requires.append(get_require_version('trytond_%s' % dep))
 requires.append(get_require_version('trytond'))
 
+tests_require = []
+
 setup(name=name,
     version=version,
     description='Tryton module with product attributes',
@@ -113,6 +115,9 @@ setup(name=name,
     license='GPL-3',
     python_requires='>=3.6',
     install_requires=requires,
+    extras_require={
+        'test': tests_require,
+        },
     zip_safe=False,
     entry_points="""
     [trytond.modules]

modules/product_measurements/setup.py

@@ -46,6 +46,8 @@ for dep in info.get('depends', []):
         requires.append(get_require_version('trytond_%s' % dep))
 requires.append(get_require_version('trytond'))
 
+tests_require = []
+
 setup(name=name,
     version=version,
     description='Tryton module to add measurements to product',
@@ -114,6 +116,9 @@ setup(name=name,
     license='GPL-3',
     python_requires='>=3.6',
     install_requires=requires,
+    extras_require={
+        'test': tests_require,
+        },
     zip_safe=False,
     entry_points="""
     [trytond.modules]