update libusb1 dependency

[prusnak]

In setup.py we have 'libusb1>=1.7,<3',, but recently version 3.x has been released.

We should relax the dependency to 'libusb1>=1.7,<4',

The reason why the major version is bumped is that Py 3.6 compatibility has been dropped (as Py3.6 is EOL). See https://github.com/vpelletier/python-libusb1#300