python: Use __name__ instead of func_name for Py3 compat

Change-Id: I62a9685b4bce7e9012bc65309fcafe26135fde6d
Signed-off-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/15997
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>