Bring back FromPyObject implementations for structs wrapping &str references once the second lifetime is added to the trait in pyo3

[ljeub-pometry]

With the Bound changes in pyo3, it is currently no longer possible to get a &str reference with a named lifetime in FromPyObject implementations which means we had to introduce a bunch of extra wrapper structs to deal with this. Once PyO3/pyo3#4390 is merged and released, we should remove those again.