Add a convenient shortcut `Device.arch` that returns the CC value as a string

[leofang]

btw I get annoyed having to do a silly string concat like this everywhere. Do you think it'd be better if we have a shortcut Device.arch that returns this string?

Originally posted by @leofang in #720 (comment)

This refers to the usage like this that we have everywhere:

arch = "".join(f"{i}" for i in dev.compute_capability)

I want to be able to just write

arch: str = dev.arch