Add empty extras test to modules without needs

Since version 4.36.0 tox is validating extras option.

modules/account_de_skr03/setup.py

@@ -50,6 +50,8 @@ 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 German chart of accounts SKR03',
@@ -100,6 +102,9 @@ setup(name=name,
     license='GPL-3',
     python_requires='>=3.9',
     install_requires=requires,
+    extras_require={
+        'test': tests_require,
+        },
     zip_safe=False,
     entry_points="""
     [trytond.modules]