sfs3store.py has a hard dependency on aiobotocore

[xhochy]

This leads to the problem that we cannot create a filesystem store without having aiobotocore installed. This should be an optional dependency though.

 │ + python -c 'from minimalkv import get_store; get_store("hfs", path=".")'
 │ Traceback (most recent call last):
 │   File "<string>", line 1, in <module>
 │   File "$PREFIX/lib/python3.9/site-packages/minimalkv/_get_store.py", line 232, in get_store
 │     store = create_store(type, params)
 │   File "$PREFIX/lib/python3.9/site-packages/minimalkv/_store_creation.py", line 32, in create_store
 │     return _create_store_hfs(type, params)
 │   File "$PREFIX/lib/python3.9/site-packages/minimalkv/_store_creation.py", line 126, in _create_store_hfs
 │     from minimalkv._hstores import HFilesystemStore
 │   File "$PREFIX/lib/python3.9/site-packages/minimalkv/_hstores.py", line 11, in <module>
 │     from minimalkv.net.s3fsstore import S3FSStore
 │   File "$PREFIX/lib/python3.9/site-packages/minimalkv/net/s3fsstore.py", line 12, in <module>
 │     from minimalkv.net._aws_refreshable_session import (
 │   File "$PREFIX/lib/python3.9/site-packages/minimalkv/net/_aws_refreshable_session.py", line 9, in <module>
 │     from aiobotocore.credentials import (
 │ ModuleNotFoundError: No module named 'aiobotocore'