[clang-tools-extra] [clang-tidy][NFC] fix add_new_check python3.8 incompatibility (PR #107871)

[Nicolas van Kempen via cfe-commits]

nicovank wrote:

Exactly, IMO the move to 3.9 minimum should be in 6-12 months to leave a bit of time after EOL. 3.9 also has subscriptable `tuple`, `list` and `dict` which is nice. 

https://github.com/llvm/llvm-project/pull/107871