[llvm] Fix update-test-checks.py on Python 3.6 (PR #93116)

[Eli Friedman via llvm-commits]

efriedma-quic wrote:

> Do we really need to support it?

I could probably upgrade my Python my builds use personally.  But the documentation still says 3.6, and the build system still enforces 3.6, so we should support 3.6 as long as that's true.  If you want to change the minimum, please start a Discourse thread.

https://github.com/llvm/llvm-project/pull/93116