npy: fix parsing of complex dtypes

[japm48]

An exported .npy file with complex single float dtype fails to be loaded with FileFormat("unrecognized descr c8") (similarly with c16 for complex double).
According to the documentation, c{size} are the only values that should be parsed as complex. I could not find any reference of F or D as introduced in #468.

[LaurentMazare]

I feel that we should keep the support for F and D just in case as it's for reading only and is not used by any other dtype. I can find some references to it here for example though it's hard to tell if it can end up in a .npy file.

[japm48]

Thanks for the research. I updated the PR accordingly.