Revert "REF: use astype_nansafe in Index.astype"
Reverts pandas-dev/pandas#38518
may also close #38592, see https://github.com/pandas-dev/pandas/issues/38592#issuecomment-748691750 ~this is for 1.3?~ nvm i see
 @jbrockmendel ok here ? Yep

On Mon, Dec 21, 2020 at 8:41 AM Jeff Reback <notifications@github.com>
wrote:

> @jbrockmendel <https://github.com/jbrockmendel> ok here ?
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 > may also close #38592, see [#38592 (comment)](https://github.com/pandas-dev/pandas/issues/38592#issuecomment-748691750)

hmm, maybe not.

```
       before           after         ratio
     [d3c52e4a]       [15daa4c5]
     <master>         <revert-38518-ref-astype-index>
-        9.82±1ms       7.09±0.1ms     0.72  stat_ops.FrameOps.time_op('sem', 'Int64', 0)

SOME BENCHMARKS HAVE CHANGED SIGNIFICANTLY.
PERFORMANCE INCREASED.
``` thanks @simonjayhawkins 

@jbrockmendel might want to rebase any PRs which touch this 