Fix documentation of argument types for anndata.concat
Fix documentation of argument types for anndata.concat

Types were previously not showing up.
This was due to sphinx_autodoc_typehints failing to process `Optional` and `collections.Mapping[str, AnnData]` as valid typehints (since they are not).
However, instead of creating any warning or error, this just silently passed and did nothing.

theislab/anndata