Returning string from Python callback to Go - and PyBytes_AsString() SIGSEGV

Hi,

Thanks for the great tool!

# Description

Trying to return a string from Python callback to Go - and getting SIGSEGV:

```
> python test.py
fs.CallBack(22, cbfun)...
in python cbfun: FieldI: 42 FieldS: str field ival: 22 sval: str field
fs.CallBackIf(22, cbfunif)...
in python cbfunif: FieldI: 42 FieldS: str field ival: 22 ifval: str field
fs.CallBackRval(22, cbfunrval)...
in python cbfunrval: FieldI: 42 FieldS: str field ival: 22 ifval: str field
```
(nothing more in output, literally)

Gdb shows more:

```
(gdb) back
#0 0x000055b7519d2627 in PyErr_Format ()
#1 0x000055b7519003a6 in ?? ()
#2 0x00007f330e0d0dab in _cgo_86d28a1f6e85_Cfunc_PyBytes_AsString () from /app/out/_funcs.cpython-310-x86_64-linux-gnu.so
#3 0x00007f330e098c41 in runtime.asmcgocall () at /usr/local/go/src/runtime/asm_amd64.s:923
#4 0x000000c000006700 in ?? ()
#5 0x00007ffee5d3e9a0 in ?? ()
#6 0x000055b763a8eee0 in ?? ()
#7 0x000000c000044008 in ?? ()
#8 0x00007ffee5d3ea58 in ?? ()
#9 0x00007f330e0a0c21 in crosscall2 () at /usr/local/go/src/runtime/cgo/asm_amd64.s:43
#10 0x00007f330e0d0c07 in funcs_FunStruct_CallBackRval () from /app/out/_funcs.cpython-310-x86_64-linux-gnu.so
#11 0x00007f330e0d5176 in _wrap__funcs_funcs_FunStruct_CallBackRval () from /app/out/_funcs.cpython-310-x86_64-linux-gnu.so
#12 0x000055b7519cbdae in ?? ()
#13 0x000055b7519c252b in _PyObject_MakeTpCall ()
#14 0x000055b7519bb16b in _PyEval_EvalFrameDefault ()
#15 0x000055b7519cc6ac in _PyFunction_Vectorcall ()
#16 0x000055b7519b4b2b in _PyEval_EvalFrameDefault ()
#17 0x000055b7519b1096 in ?? ()
#18 0x000055b751aa6f66 in PyEval_EvalCode ()
#19 0x000055b751ad1e98 in ?? ()
#20 0x000055b751acb79b in ?? ()
#21 0x000055b751ad1be5 in ?? ()
#22 0x000055b751ad10c8 in _PyRun_SimpleFileObject ()
#23 0x000055b751ad0d13 in _PyRun_AnyFileObject ()
#24 0x000055b751ac370e in Py_RunMain ()
#25 0x000055b751a99dfd in Py_BytesMain ()
#26 0x00007f330e733d90 in ?? () from /lib/x86_64-linux-gnu/libc.so.6
#27 0x00007f330e733e40 in __libc_start_main () from /lib/x86_64-linux-gnu/libc.so.6
#28 0x000055b751a99cf5 in _start ()
```

Trying _examples/funcs/
Just 2 changes to ret type string:

funcs.go:
```
-func (fs *FunStruct) CallBackRval(i int, fun func(fs *FunStruct, i int, v interface{}) bool) {
+func (fs *FunStruct) CallBackRval(i int, fun func(fs *FunStruct, i int, v interface{}) string) {
```

test.py:
```
in def cbfunrval(afs, ival, ifval):
- return True
+ return "some string"
```

# Versions
Gopy:
commit 1d7f3a2d29bd9f3921ff640f8e4e8908170ff3bf (HEAD -> master, tag: v0.4.10, origin/master, origin/HEAD)

Python 3.10.12 (main, Jul 29 2024, 16:56:48) [GCC 11.4.0] on linux
go version go1.23.2 linux/amd64

# The Reason
Generated out/funcs.go:

funcs_FunStruct_CallBackRval is generated to pass _fcret without type checks:
```
...
_fcret := C.PyObject_CallObject(_fun_arg, _fcargs)
C.gopy_decref(_fcargs)
C.gopy_err_handle()
C.PyGILState_Release(_gstate)
return C.GoString(C.PyBytes_AsString(_fcret))
...
```

And PyBytes_AsString - panics as _fcret is not bytes.

# Workaround

But, changing ret type to bytes - and things work fine:
test.py:
```
in def cbfunrval(afs, ival, ifval):
- return "some string"
+ return b"some string"
```

```
> python test.py
fs.CallBack(22, cbfun)...
in python cbfun: FieldI: 42 FieldS: str field ival: 22 sval: str field
fs.CallBackIf(22, cbfunif)...
in python cbfunif: FieldI: 42 FieldS: str field ival: 22 ifval: str field
fs.CallBackRval(22, cbfunrval)...
in python cbfunrval: FieldI: 42 FieldS: str field ival: 22 ifval: str field
got return value: some string
fs.CallBack(32, cls.ClassFun)...
in python class fun: FieldI: 42 FieldS: str field ival: 32 sval: str field
cls.CallSelf...
in python class fun: FieldI: 42 FieldS: str field ival: 77 sval: str field
fs.ObjArg with nil
got nil
fs.ObjArg with fs
ofs FieldI: 42 FieldS: str field
OK
```

# Fix
So, need to test-cover PyBytes_AsString calls with more types.

# Thank you!