Disabling netlabs-rel worked to install Python3. Seems every time I've installed it, the procedure has changed.
Building with your configure line and ninja, the build dies here,
[1114/23544] COPY gen/third_party/devtools-frontend/src/front_end/third_party/acorn/acorn.js resources/inspector/third_party/acorn/acorn.js
FAILED: resources/inspector/third_party/acorn/acorn.js
ln -f gen/third_party/devtools-frontend/src/front_end/third_party/acorn/acorn.js
resources/inspector/third_party/acorn/acorn.js 2>/dev/null || (rm -rf resources/inspector/third_party/acorn/acorn.js && cp -af gen/third_party/devtools-frontend/src/front_end/third_party/acorn/acorn.js resources/inspector/third_party/acorn/acorn.js)
cp: cannot stat 'gen/third_party/devtools-frontend/src/front_end/third_party/acorn/acorn.js': No such file or directory
I find 3 acorn.js in my tree, at 204KB,
L:/work/qt6/qt6-webengine-os2/src/3rdparty/chromium/third_party/devtools-frontend/src/front_end/third_party/acorn/package/dist/acorn.js
at 200KB,
L:/work/qt6/qt6-webengine-os2/src/3rdparty/chromium/third_party/devtools-frontend/src/node_modules/acorn/dist/acorn.js
and a 993 byte one at,
L:/work/qt6/qt6-webengine-os2/src/3rdparty/chromium/third_party/devtools-frontend/src/node_modules/recast/parsers/acorn.js
Not sure where it is supposed to copy, no res/ that I see here.