use log.deprecated in py2vs3 module

[boegel]

@branfosj for easybuilders#4229

This yields a better warning message, which will automatically turn into an error message with EasyBuild >= 6.0:

$ eb Perl.eb -D
WARNING: Deprecated functionality, will no longer work in v6.0: Using py2vs3 is deprecated, since EasyBuild no longer runs on Python 2.; see http://easybuild.readthedocs.org/en/latest/Deprecated-functionality.html for more information

$ eb Perl.eb --deprecated 6.0
ERROR: Failed to process easyconfig /Users/kehoste/work/easybuild-easyconfigs/easybuild/easyconfigs/p/Perl/Perl-5.36.0-GCCcore-12.2.0.eb: DEPRECATED (since v6.0) functionality used: Using py2vs3 is deprecated, since EasyBuild no longer runs on Python 2.; see http://easybuild.readthedocs.org/en/latest/Deprecated-functionality.html for more information

I'll open a separate framework PR to fix the outdated link to the docs (although the current link still works, it gets auto-redirected to https://docs.easybuild.io/deprecated-functionality)