gthumb build-requires python 3 and shebangfixes

: FAILED: gthumb/AUTHORS.tab
: /wrkdirs/usr/ports/graphics/gthumb/work/gthumb-3.10.0/gthumb/make-authors-tab.py /wrkdirs/usr/ports/graphics/gthumb/work/gthumb-3.10.0/AUTHORS gthumb/AUTHORS.tab
: env: python3: No such file or directory
: ninja: build stopped: subcommand failed.

Approved by: just-fix-it