Bogus "is either too old or uses too old a Python version" from docs/meson.build

[Markus Armbruster]

I just chanced on this one:

    Program sphinx-build-3 found: YES
    ../docs/meson.build:30: WARNING: /usr/bin/sphinx-build-3 is either too old 
or uses too old a Python version

    ../docs/meson.build:33:6: ERROR: Problem encountered: Install a Python 3 
version of python-sphinx

    A full log can be found at 
/work/armbru/qemu/bld-x86/meson-logs/meson-log.txt

    ERROR: meson setup failed

My sphinx-build-3 is just fine, the problem is caused by me changing my
tree so that

1. qapi-gen.py fails (because I messed up), and

2. make re-runs configure.

Perhaps the test for a working sphinx-build-3 could be made a bit more
robust.