Speedup dependencies dtypes

[hagenw]

Speedup audb.Dependencies by using object dtype for strings instead of string.

This decision is based on the benchmark results comparing storing string as object or string, or using pyarrow dtypes (for discussion on benchmark see #365). One can see that we get slight improvments when strings as object and not as string, whereas using pyarrow dtypes slows down row based processing.

TODO: add a test what happens when we load existing db.pkl from cache that are using sting as dtype.

[hagenw]

This is superseeded by #372