meson complains about "broken python installation"

[Peter Maydell]

At some point in the last month a meson change landed, and now
when meson runs it complains:

 WARNING: Broken python installation detected. Python files installed
 by Meson might not be found by python interpreter.

There's nothing in the meson log here that says what it's
complaining about:


Build machine cpu: x86_64
Host machine cpu family: x86_64
Host machine cpu: x86_64
Target machine cpu family: x86_64
Target machine cpu: x86_64
Program sh found: YES (/bin/sh)
Program python3 found: YES (/usr/bin/python3)
WARNING: Broken python installation detected. Python files installed
by Meson might not be found by python interpreter.
Program bzip2 found: YES (/bin/bzip2)
Sanity testing C++ compiler: c++ -m64 -mcx16
Is cross compiler: False.

I didn't pass any --python option to configure, and this is
a fresh build in a newly created build directory. Host is
x86-64 Ubuntu 18.04.6 LTS (bionic). /usr/bin/python is
Python 2.7.17 (which we do not use) and /usr/bin/python3
is Python 3.6.9.

Any idea what meson is complaning about ?

thanks
-- PMM