TYP: some type annotations for interpolate
pre-cursor to #34628
Yea I don’t think its that important so if a lot of effort just revert

> On Jun 7, 2020, at 10:35 AM, Simon Hawkins <notifications@github.com> wrote:
> 
> 
> @simonjayhawkins commented on this pull request.
> 
> In pandas/core/missing.py <https://github.com/pandas-dev/pandas/pull/34631#discussion_r436385068>:
> 
> > @@ -92,8 +94,7 @@ def clean_fill_method(method, allow_nearest=False):
>      return method
>  
>  
> -def clean_interp_method(method, **kwargs):
> -    order = kwargs.get("order")
> +def clean_interp_method(method: str, order: Optional[int] = None, **kwargs) -> str:
> it doesn't appear that we pass the kwargs through to np.interp (left, right and period) but do for scipy.interpolate.interp1d
> 
> did you mean kwargs not used in this function? That's true, but would need to do order = kwargs.get("order") in the calling functions if I remove kwargs here. I don't see that as an improvement.
> 
> should I revert this change?
> 
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 Thanks @simonjayhawkins 