[
	{bla: ala}
]
 ---->
[
	{bla: ala
bla

-----------------

with fileinput.FileInput(filename,
                         inplace = True, backup ='.bak') as f:

    for line in f:
        if "}\n]" in line:
            print(line.replace("}\n]",
                               "\nbla"), end ='')
        else:
            print(line, end ='')