Support for pyyaml 6

[kbroch-rivosinc]

Currently only support pyyaml==5.2

[olofk]

It does seem to work with newer versions in practice, despite the strict version check. In order to run riscof with SERV I'm using a forked version of riscv-config where I just changed this to pyyaml>=5.2 and it has worked for me so far.

[shareefj]

+1