meson: stop looking for 'sphinx-build-3'

meson: stop looking for 'sphinx-build-3'

Once upon a time, "sphinx-build" on certain RPM platforms invoked
specifically a Python 2.x version, while "sphinx-build-3" was a distro
shim for the Python 3.x version.

These days, none of our supported platforms utilize a 2.x version, and
those that still have 'sphinx-build-3' make it a symbolic link to
'sphinx-build'.  Not searching for 'sphinx-build-3' will prefer
pip/venv installed versions of sphinx if they're available.

This adds an extremely convenient ability to test document building
ability in QEMU across multiple versions of Sphinx for the purposes of
compatibility testing.

Signed-off-by: John Snow <jsnow@redhat.com>
Message-Id: <20230221012456.2607692-6-jsnow@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>