Remove python 3.8 dependency (Big Sur)

[soheil]

Uses the default python3 currently installed on the system since the current pre-release OSXI (Big Sur) does not build python 3.8, see #56791

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[mistydemeo]

Sorry, but we're not going to be able to merge this. It may take us a few days to get a fix in for the Python 3.8 build, so please be patient in the meantime!

[iMichka]

Regarding system python 3: is this still the CLT shim like on Catalina?

We might stay on brewed python3 too, but we need to reach a consensus on that with the other maintainers.

[Bo98]

I checked and it is, though it seems to work with Xcode too.