Change Future to DeprecationWarning for make_block_same_class
xref https://github.com/pandas-dev/pandas/pull/19434

@jreback I didn't want to further discuss on the PR, so let's do that here :-)

By having it as a FutureWarning, we only annoy users, and the fastparquet developers are already aware of it. 
BTW, we do exactly the same for pyarrow's 'misuse' of internal API, we added a deprecationwarning for them.