adb: switch test_adb.py to python3.

Currently, test_adb.py doesn't work on Windows, because we're selecting
on a pipe, which doesn't work on Windows. Python 3.5 adds socketpair
support to Windows, so switch over to Python 3 to make it so we can use
that.

Test: python3 test_adb.py
Change-Id: I0fbd1bca0b28324658831d5ef8ee08aefe2b0596