When binding to the Python C API via

# pypdfium2 ctypesgen
ctypesgen --system-headers python3.11/Python.h --all-headers -l python --dllclass pythonapi -o ctypes_python.py

or

# legacy ctypesgen
# then manually replace CDLL with PyDLL, or better load_library("python3") with ctypes.pythonapi (this is to get the GIL & error checking correct)
ctypesgen /usr/include/python3.11/Python.h --all-headers -l python3 -o ctypes_python.py

I get the following strange exception:

Traceback (most recent call last):
  File "/home/me/projects/third_party/ctypesgen_orig/ctypes_python.py", line 15940, in <module>
    struct__typeobject._fields_ = [
    ^^^^^^^^^^^^^^^^^^^^^^^^^^^
AttributeError: _fields_ is final

struct__typeobject is the ctypesgen definition alias of PyTypeObject and _typeobject, which apparently is some sort of complex forward-declared struct with direct and indirect self-references.
But that doesn't yet explain the exception. In particular, it works if we isolate just this struct and its dependencies by adding --symbol-rules 'if_needed=\w+' yes=PyTypeObject to the pypdfium2 ctypesgen call.¹

Attached are the two samples complete.py (broken) and isolated.py (passing): Samples.zip

Excluding PyTypeObject and dependents via --symbol-rules never=... would only be a workaround, and is undesirable because this is a core object.
I guess a reasonable workaround might be to make it opaque by linking in a spoof definition, though (as suggested in #20 (comment)).