pybind/mgr/cephadm/inventory: cast variable to expected type

pybind/mgr/cephadm/inventory: cast variable to expected type

to silence mypy warnings like:

cephadm/module.py:55: note: In module imported here,
cephadm/__init__.py:6: note: ... from here:
cephadm/inventory.py: note: In member "load" of class "SpecStore":
cephadm/inventory.py:130: error: Invalid index type "str" for "str"; expected type "Union[int, slice]"
cephadm/inventory.py:131: error: Invalid index type "str" for "str"; expected type "Union[int, slice]"

Signed-off-by: Kefu Chai <kchai@redhat.com>
(cherry picked from commit cb3a50cd3626d6a67beb39d2c9dd897f43379d70)