TYP: Index.join, get_indexer, reindex
@simonjayhawkins this is technically incorrect bc im specifying returned dtypes as np.intp.  Is there a valid way to do this?  In working in core.internals trying to figure out whether we want int64 or intp I had to track all of these down to be sure.  Would prefer to avoid re-doing this in a few weeks when I forget.
> @simonjayhawkins this is technically incorrect bc im specifying returned dtypes as np.intp. Is there a valid way to do this? In working in core.internals trying to figure out whether we want int64 or intp I had to track all of these down to be sure. Would prefer to avoid re-doing this in a few weeks when I forget.

will need to use comments to retain this info for now because `"ndarray" expects no type arguments, but 1 given` in numpy 1.20.x

I'll start another branch shortly, in the same vien as #36092 checking against numpy master to get visibility of upcoming changes
 OK, I guess I'll mothball this to clear the queue, will re-open when these are supported