TYP: Typing changes for ExtensionArray.astype
Minimal (hopefully) typing changes for `ExtensionArray.astype` .  

@simonjayhawkins can you review, please? This is now failing mypy with master merged. can you merge master and fixup. @simonjayhawkins  pushed changes a week ago, so looking forward to a new review. This pull request is stale because it has been open for thirty days with no activity. Please update or respond to this comment if you're still interested in working on this. @jbrockmendel @simonjayhawkins got marked stale due to lack of review/approval.... > Thanks @Dr-Irv
> 
> is it worth posting the results of a test file with reveal_types for this?

Can you point me to an example? @simonjayhawkins ping... pinging @simonjayhawkins  @Dr-Irv thanks for the patience. can you rebase

@simonjayhawkins if you can have a look weekly ping @simonjayhawkins  another ping @simonjayhawkins  biweekly ping @simonjayhawkins  @Dr-Irv ok if you can merge master and make sure still passing (i think it was last) time, will merge these. @jreback all green except for a Windows timeout on one of the tests can you rebase again just to be sure (as merged the other)

cc @twoertwein  @jreback all green after merge with the delete/searchsorted changes thanks @Dr-Irv 