stbt.py: [pylint 1.0] use new-style classes throughout
stbt.py: [pylint 1.0] use new-style classes throughout

Pylint 1.0 insists on new-style classes, meaning that custom classes
must inherit from either `object` or another new-style class.

LewisHaley/stb-tester,martynjarvis/stb-tester,wmanley/stb-tester,wmanley/stb-tester,wmanley/stb-tester,martynjarvis/stb-tester,LewisHaley/stb-tester,LewisHaley/stb-tester,stb-tester/stb-tester,LewisHaley/stb-tester,martynjarvis/stb-tester,martynjarvis/stb-tester,LewisHaley/stb-tester,wmanley/stb-tester,stb-tester/stb-tester,wmanley/stb-tester,martynjarvis/stb-tester,martynjarvis/stb-tester,stb-tester/stb-tester,martynjarvis/stb-tester,stb-tester/stb-tester,LewisHaley/stb-tester,LewisHaley/stb-tester