Upcoming incompatibility with CPython 3.15

[tacaswell]

python/cpython#141108 that removes an enum value https://github.com/python/cpython/pull/141108/files#diff-1541528aa8ba238fa13865ed284b8b100b9f35bfea78237cfd2f5506715d8367L27

g++ -fPIC -I/home/tcaswell/.virtualenvs/bisect/include -I/home/tcaswell/.pybuild/bisect/include/python3.15 -c src/greenlet/
greenlet.cpp -o build/temp.linux-x86_64-cpython-315/src/greenlet/greenlet.o
In file included from /home/tcaswell/.pybuild/bisect/include/python3.15/internal/pycore_backoff.h:12,
                 from /home/tcaswell/.pybuild/bisect/include/python3.15/internal/pycore_code.h:11,
                 from /home/tcaswell/.pybuild/bisect/include/python3.15/internal/pycore_interpframe.h:8,
                 from src/greenlet/TGreenlet.hpp:35,
                 from src/greenlet/greenlet_internal.hpp:18,
                 from src/greenlet/greenlet.cpp:19:
src/greenlet/TGreenlet.cpp: In member function ‘void greenlet::Greenlet::expose_frames()’:
src/greenlet/TGreenlet.cpp:636:41: error: ‘FRAME_OWNED_BY_CSTACK’ was not declared in this scope; did you mean ‘FRAME_OWNED_BY_THREAD’?
  636 |             assert(iframe_copy.owner != FRAME_OWNED_BY_CSTACK);
      |   

If -DNDEBUG is passed then greenlet compiles with only warnings. I had -fpermissive set in my $CXXFLAGS which seems to cause something in the build chain to drop -DNDEBUG . There may be a better way to trigger this, but it setting CXXFLAGS='-fpermissive' is key to reproducing (it may be setting anything is enough?).

Given that the assert is checking the variable has a value that no longer exists I think it should be safe to delete the line.