REF: use astype_array instead of astype_nansafe
Trying to ensure we have only one astype behavior.

The BaseParser change _could_ change behavior if we can get there with dt64/td64 data (that includes at least one NaT) and an integer dtype, but we don't have any test cases that get there with dt64/td64 and it isn't clear to me if it is possible.  If it is, the non-raising behavior here is more consistent with our other behavior.
Thanks @jbrockmendel 