[travis] Make tox use Python 3.7 for the travis's lint stage
[travis] Make tox use Python 3.7 for the travis's lint stage

Because we make use of some mock methods in module `test_util` (eg: `mock.assert_called_once`) which were introduced in python 3.6, so if the system python of the test system is lower than 3.6, then the tox tests will fail

rnixx/python-for-android,kronenpj/python-for-android,germn/python-for-android,rnixx/python-for-android,kivy/python-for-android,rnixx/python-for-android,kivy/python-for-android,PKRoma/python-for-android,kivy/python-for-android,germn/python-for-android,kronenpj/python-for-android,germn/python-for-android,rnixx/python-for-android,kivy/python-for-android,PKRoma/python-for-android,PKRoma/python-for-android,germn/python-for-android,kronenpj/python-for-android,PKRoma/python-for-android,rnixx/python-for-android,kivy/python-for-android,germn/python-for-android,rnixx/python-for-android,germn/python-for-android,PKRoma/python-for-android,kronenpj/python-for-android,kronenpj/python-for-android