Just FWIW, you could also access the data using the rpm python bindings, its already used for similar purposes in get_build_arches_from* etc. Eg

>>> import rpm
>>> spec = rpm.spec('popt.spec')
>>> spec.sourceHeader['CHANGELOGTEXT'][0]
'- Rebuilt for https://fedoraproject.org/wiki/Fedora_41_Mass_Rebuild'

Thanks, @pmatilai, your approach looks cleaner!
I already merged the fix, but I will consider changing it.
What I am afraid of a little are the other arguments of the current rpm query command. It has defined macros (self.rpmdefines) and macro _changelog_trimtime undefined. It is a long time ago and I don't remember what would the consequences of removing this stuff :-).
At least _changelog_trimtime was the subject of another issue in the past.
https://github.com/rpm-software-management/rpm/issues/1301

Yeah those defines do matter, and also one needs to be aware that parsing the spec will affect the state of the calling process - macros defined/undefined by the spec etc. Which is something you don't need to worry about with just calling rpmspec in an external process. Probably the reason SpecFile class in rpkg does it the way it does.