REGR: merge no longer accepts a list for suffixes keyword
see https://github.com/pandas-dev/pandas/pull/34208#issuecomment-643598091

The linked PR changed the `merge` routines to now only accept tuples for the `suffixes` keyword, and no longer general sequence.

This is a breaking change (eg it breaks some GeoPandas functions), but actually also changing documented behaviour: the `merge` docstring indeed says "tuple", but eg `merge_asof` still says "tuple or list". And in our user guide, we were ourselves using a list in one example, and it explicitly says "tuple or list" (see https://pandas.pydata.org/pandas-docs/version/1.0/user_guide/merging.html#overlapping-value-columns)

In general, I am certainly fine with restricting the set of accepted types (eg to avoid the confusing issue with sets, the original report), but this is 1) breaking a documented behaviour and 2) a change that could also easily be done with a deprecation warning. 
And given our versioning policy, if it is easy to do a change with a deprecation, then we should try to do that, IMO.
