Memory Leak on MacOS

Calling the `keyring.get_password()` function repeatedly on MacOS results in leaked memory. The problem appears to be in [find_generic_password()](https://github.com/jaraco/keyring/blob/f3e711d237ad94314dd5738667ee5ed6bc2f33e6/keyring/backends/macOS/api.py#L142C1-L159C30), where the memory allocated by `SecItemCopyMatching()` is never released. I spent some time trying to fix the issue, but it was dragging me further into the world of CoreFoundation internals than I had time to go.

I don't know how serious the problem is in practice, but using memray I was seeing a non-negligible memory allocation (~30MB) hanging around in my application, attributable to this function.

Thank you so much for your work maintaining this library! If I have more time soon I might return to the issue and attempt a fix.

Start in keyring/backends/macOS/api.py at find_generic_password(), especially the SecItemCopyMatching() call and the surrounding CoreFoundation handling. Use the reported repeated keyring.get_password() scenario and memray to investigate; done means the allocation no longer accumulates while the lookup continues to work.

Written by the indexing model from the issue text.