devel/llvm*: Consistently use python:3.7+

llvm12 was inappropriately constrained to 3.6-3.9 breaking the ability
to set 3.10 as the default. llvm11 just specified "python", but there's
no need to keep 2.7 support around. The rest had obsolete 3.6+ entries.

No functional change: This is not expected to change the version of
python used.

PR: 270978
MFH: 2023Q2
Reported by: michael.osipov@siemens.com
Sponsored by: DARPA