Timescale should also support fs

[alexforencich]

The timescale parameter is currently checked with a regex like so:

if timescale is None or re.fullmatch("\\d+[npu]?s/\\d+[npu]?s", timescale):

Unfortunately, this regex is wrong as fs (femtoseconds) is also a valid timescale. Seems ms has the same issue. It's probably worth checking against relevant documentation to make sure this regex doesn't need to be updated again.

[themperek]

Can you make PR? If not I will try to fix this.