graph-build-time: fix code style

Fix these warnings:
E201 whitespace after '['
E202 whitespace before ']'
E302 expected 2 blank lines, found 1
E305 expected 2 blank lines after class or function definition, found 1

Ignore these warnings:
E402 module level import not at top of file

Signed-off-by: Ricardo Martincoski <ricardo.martincoski@gmail.com>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Yann E. MORIN <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>