BUG: fix nanmedian for CoW without bottleneck
Fixing a failing test that turned up in https://github.com/pandas-dev/pandas/pull/55732. 
This PR specifically fixes `pandas/tests/frame/test_reductions.py::test_reduction_axis_none_returns_scalar[float64-True-median]`, i.e. calling `df.median(axis=None)`, for builds that don't have bottleneck installed.

If the underlying data of the DataFrame or Series is float64 data, the `values` inside `nanops.nanmedian(..)` will be the original values from the DataFrame, and with CoW set to be read-only (for other dtypes, we cast to float64 inside the function, and so that will always give a copy). 

And then we do `values[mask] = np.nan`, which raises an error if values is read-only. In practice, this didn't give any problems with mutating the original DataFrame, because if you start with float64 values, the `mask` always corresponds with NaN values already (so we were actually mutating the calling dataframe, just without any effect).
thx @jorisvandenbossche 