[openssl/tools] 9bf20f: update ghmerge to python3

[Neil Horman]

  Branch: refs/heads/master
  Home:   https://github.com/openssl/tools
  Commit: 9bf20f9e8f9022ab1104bd3a4cdc74ad4ade0b87
      https://github.com/openssl/tools/commit/9bf20f9e8f9022ab1104bd3a4cdc74ad4ade0b87
  Author: Neil Horman <nhorman at openssl.org>
  Date:   2024-01-18 (Thu, 18 Jan 2024)

  Changed paths:
    M review-tools/ghmerge

  Log Message:
  -----------
  update ghmerge to python3

using ghmerge currently creates errors for me, in the form of inability
to load various python packages, as fedora moved to python3 several
releases ago

propose updating the use of python to python3 here to support recent
distributions.

Reviewed-by: Hugo Landau <hlandau at openssl.org>
Reviewed-by: Tomas Mraz <tomas at openssl.org>
(Merged from https://github.com/openssl/tools/pull/176)