Importing rubicon.objs hangs on pypy

### Describe the bug

When I try to import `rubicon.objc` using Pypy, the process hangs while it allocates more and more memory. I traced this down to this line, and it seems to be related to the `PyDictObject`:

https://github.com/beeware/rubicon-objc/blob/6c3d6b663b2a754506c3115dc37a769f832b6395/src/rubicon/objc/ctypes_patch.py#L121-L132

With "traced it down" I mean that I put `1/0` in the code to make it raise an exception. If the it hung instead of raising the exception, then the hangup occurred bedore it. The code above is where it hangs. And if I comment the field with the `PyDictObject`, then it does not hang, so it looks like it's related to that.

I tried with pypy3.8-7.3.11 and pypy3.9-7.3.11, both installed via pyenv.

### Steps to reproduce

Run `import ubicon.objc` in a pypy interpreter. The process hangs, does not respond to ctrl-c.

### Expected behavior

I would expect the import to work :)

### Screenshots

_No response_

### Environment

MacOS M1 pro Sonoma 14.3

### Logs

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### Additional context

_No response_

Start with src/rubicon/objc/ctypes_patch.py at lines 121-132 and reproduce the hang by running import rubicon.objc under the reported PyPy versions. Investigate the PyDictObject handling there and verify that the import completes without unbounded memory growth or becoming unresponsive.

Written by the indexing model from the issue text.