[dexter] Require python >= 3.6

The documentation says we need python >= 3.6. Running it with an older
version, we get verbose output from python interpreter.
This patch fixes that as:

  $ python2 dexter.py list-debuggers
  You need python 3.6 or later to run DExTer

Differential Revision: https://reviews.llvm.org/D78621

GitOrigin-RevId: 871388e384dc39bf7c69866ab851e2fbaff8d4c8
1 file changed