BUG: fix online ewma with CoW
Fixing some failing tests that turned up in https://github.com/pandas-dev/pandas/pull/55732. This PR specifically fixes `pandas/tests/window/test_online.py::TestEWM::test_online_vs_non_online_mean` for Copy-on-Write enabled.

The numba function takes the underlying frame's array of values, and then writes into those values:

https://github.com/pandas-dev/pandas/blob/c36e302c7b30bc7945119a77f255d0302307847e/pandas/core/window/online.py#L80

(this `weighted_avg` is a view in the passed values, from `weighted_avg = values[0]`)

That currently doesn't cause a bug, because before getting the values from the frame and passing it to the numba func, we cast to float64:

https://github.com/pandas-dev/pandas/blob/c36e302c7b30bc7945119a77f255d0302307847e/pandas/core/window/ewm.py#L1071

This `astype()` call currently always returns a copy by default (even if you already have all float64 columns), but with CoW enabled, this avoids making a copy. Currently, with CoW enabled, the numba func fails because of the numpy array being marked as read-only. But if we wouldn't manually copy the data and just ignore this read-only flag, we would be mutating the calling dataframe (so marking as read-only prevented a new bug here!).
Thanks @jorisvandenbossche 