new h5py version doesn't support numerical event labels

It appears that with h5py version 3.0.0 there is some additional type control, and creating a neo.Event with numerical labels and saving it as .nix results in an error `Can't implicitly convert non-string objects to strings` (using neo 0.9.0, nixio 1.5.0b6, h5py 3.1.0). However, this was still possible when using h5py 2.10.0 and the same neo and nixio versions.
This may be the desired behavior as the neo documentation also says to use "(numpy.array 1D dtype=’U’ or list)" for event labels. Otherwise, for more flexibility, the nixio could try to convert labels itself and raise its own warnings or errors.

Reproduce saving a neo.Event with numerical labels using h5py 3.1.0, comparing it with h5py 2.10.0 and the stated neo 0.9.0 and nixio 1.5.0b6 versions. No source file or test is named; inspect the label serialization path and clarify whether conversion or an explicit error is expected, then add coverage for the chosen behavior.

Written by the indexing model from the issue text.