usb: add pyserial to package dependencies.

Author: mattthebaker

setup.py

@@ -77,7 +77,8 @@ def run(self):
     author_email="[email protected]",
     install_requires=[
         'warble >= 1.1, < 2.0',
-        'requests'
+        'requests',
+        'pyserial'
     ],
     cmdclass={
         'build_py': MetaWearBuild,