As stated in the README you include several projects which are GPL-2.0-only. You cannot simply change the license to GPL-3.0{-only,-or-later}, because GPL-3.0 is not compatible with GPL-2.0-only.
It seems ext-utils/gprof2dot.py is the only file that contains are GPL-3.0 header. The other files are GPL-2.0-only, according to their headers.
As ext-utils/gprof2dot.py is only a script that's not compiled into the sources, it not problem that it is GPL-3.0.
I recommend to change the license information in README.md to GPL-2.0-only. Not sure if you need the confirmation of all contributors as the sources files they might have modified explicitly state that the code is GPL-2.0-only.
As stated in the README you include several projects which are GPL-2.0-only. You cannot simply change the license to GPL-3.0{-only,-or-later}, because GPL-3.0 is not compatible with GPL-2.0-only.
It seems
ext-utils/gprof2dot.pyis the only file that contains are GPL-3.0 header. The other files are GPL-2.0-only, according to their headers.As
ext-utils/gprof2dot.pyis only a script that's not compiled into the sources, it not problem that it is GPL-3.0.I recommend to change the license information in README.md to GPL-2.0-only. Not sure if you need the confirmation of all contributors as the sources files they might have modified explicitly state that the code is GPL-2.0-only.