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Parser/asdl_c.py uses `/usr/bin/env python' as an interpreter. But Python executable is not always `python'. With OpenBSD's ports, CPython's interpreters are installed as pythonX.Y. There's a variable PYTHON in the Makefile, that's what should be used.
This way make PYTHON=python2.7 works on OpenBSD.
The attached patch fixes that.
Note that the executable bit and the hashbang can be removed from asdl_c.py if the patch is applied.

I don't believe we have any desire to support unix systems that do not define 'python', and 'python3' executables in the path. If the distribution wishes to do that they'll have to patch everything to accommodate it.
That however is mostly irrelevant to this case. asdl_c.py is not needed for building python. The files it generates are already generated and included in the release tarballs, and in the release tarballs the file timestamps should be such that asdl_c.py is not invoked. (When working from a checkout this may not be true due to vcs tool file timestamp issues; you just have to touch the files manually in that case.) When the file *is* run, it is not the python being built that is used to run it, it is an already existing python. So using the PYTHON variable in the Makefile would be incorrect.

Indeed, I didn't realize that PYTHON was the name of the target interpreter and not the name a an already installed interpreter.