`time.perf_counter_ns` always a 64-bit int?

[rmlibre at riseup.net]

I'd like to capture the output of `time.perf_counter_ns()` as an 8-byte
timestamp.

I'm aware that the docs provide an undefined start value for that clock.
I'm going to assume that means it can't be expected to fit within 8
bytes. However, it would be rather convenient if it could. Does anyone
know if any such platform agnostic assumption could be made for any
fixed number of bytes, even if it isn't exactly 8 bytes?