Does this really have to be python2?

python doesn't necessarily means python2, e.g. Python 3 is default on Arch Linux.

IMO using /usr/bin/python3 would be the best option

Does it really matter though?

Does it really matter though?

It does when Python2 is going EOL in a few months and that most distributions that aren't LTS are trying to limit its presence in default installs.

python doesn't necessarily means python2, e.g. Python 3 is default on Arch Linux.

Yes, but in other cases it does mean python2. The PEP about that will probably never be implemented everywhere (and is IMO also wrong, a script has to state whether it's 2 or 3 compatible).

Does it really matter though?

Yes. Currently without patching kwin5 would not be installable as /bin/env doesn't exist leading to dependency issues.
Ignoring that, it would be the only package in a default installation pulling in python 2 again, which is a regression.

It does not need python2.
It's written and tested against python3.

My understanding was that /bin/env was the "correct" way to choose python in weird distros.

I ideally want to avoid having some cmake stuff and then reconfiguring this script. If hardcoding /usr/bin/python3 works for distros, that's more than fine with me.

My understanding was that /bin/env was the "correct" way to choose python in weird distros.

With /usr/bin/env yes, but what "python" means depends on the distro. For some it doesn't exist, for some it's py2 and for some it's py3...
Also see https://www.python.org/dev/peps/pep-0394/#for-python-script-publishers.

If hardcoding /usr/bin/python3 works for distros, that's more than fine with me.

I can't think of a situation where this would fail. /usr/bin/env python3 maybe if someone wants to use python3 from /usr/local or ~/bin.

done