BUG: read-only buffer failures in datetime parsing
PR https://github.com/pandas-dev/pandas/pull/22147 caused several "read-only buffer" errors (latest: https://github.com/pandas-dev/pandas/issues/34843). I quickly skimmed that PR, and most changes there have been fixed in the mean-time (either converted back to use ndarray or added `const`), a few are remaining, like:

`array_strptime` (and also `array_to_timedelta64` -> `to_timedelta`)
```
In [4]: arr = np.array(['15/10/2020'], dtype=object) 

In [5]: arr.flags.writeable = False   

In [6]: pd.to_datetime(arr, format="%d/%m/%Y")    
...
ValueError: buffer source array is read-only
```

and then a few in parsing.pyx, but I suppose those are generally not a problem, if the arrays are coming from out parsing code, they will never be read-only.

_Originally posted by @jorisvandenbossche in https://github.com/pandas-dev/pandas/issues/34843#issuecomment-645259205_