Allow deprecate_nonkeyword_arguments to not specify version, show class name
xref https://github.com/pandas-dev/pandas/pull/41511#discussion_r633386611

as request, ping @jreback @simonjayhawkins 

I've removed the ability to pass an integer to `allowed_arguments` as it added complexity, was only used once in the codebase anyway, and wasn't even covered by tests

Now, if `version` is None, then it'll just print "In a future version of pandas", it'll show the class name (e.g. `Series.drop_duplicates` instead of just `drop_duplicates`) and won't print `'self'` in the warning message