Reland "[python] Bump Python minimum version to 3.8 (#78828)"

This reverts commit 2dfa30d0.

This relands commit 0a6c74e2.

This patch originally caused a host of buildbot failures due to several
buildbots not having the necessary python version. That was tracked in
issue #83962, and all bots that failed in the first round have now been
updated. This is an attempt to reland the patch to see if it sticks or
if there are a number of long-running bots where this patch will cause
failures.