BUG: fixes PyPy build (do not use _PyHASH_Imag)
- [x] closes #42355
- [x] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing.html#code-standards) for how to run them
- [x] what's new entry

`_PyHASH_Imag` is not defined in PyPy, thus we define and use our own `_PandasHASH_Imag`.
