gdb: make some random Python files Python 3-compatible

I noticed that these files failed to format with Black, because they use
print without parenthesis (which isn't Python 3 compatible).

I don't know if these files are still relevant, but the change is
trivial, so here it is.

Change-Id: I116445c2b463486016f824d32effffc915b60766
3 files changed