BUG: pd.NA in str.replace raises error
>>>pd.__version__
1.1.1

I find setting pd.NA as repl of str.replace method will raise error, both on string and object.
Yet I think it may be better not to raise this error because the pd.NA indeed could be a missing string value.

>>>pd.Series(['a','b'],dtype='string').str.replace('a',pd.NA)
TypeError: repl must be a string or callable
>>>pd.Series(['a','b'],dtype='object').str.replace('a',pd.NA)
TypeError: repl must be a string or callable

And also, pd.NA can't be the pat of str.replace. Maybe reasonable to allow this.

>>>pd.Series([pd.NA,'a'],dtype='string').str.replace(pd.NA,'1')
TypeError: object of type 'NAType' has no len()