Install fails under npm 7.x

[whitefractal]

npm ERR! code 1
npm ERR! path /Users/alistair/dev/red/pngjs/node_modules/canvas
npm ERR! command failed
npm ERR! command sh -c node-gyp rebuild
npm ERR! SOLINK_MODULE(target) Release/canvas-postbuild.node
npm ERR!   CXX(target) Release/obj.target/canvas/src/Canvas.o
...
npm ERR! gyp info spawn args [ 'BUILDTYPE=Release', '-C', 'build' ]
npm ERR! In file included from ../src/Canvas.cc:20:
npm ERR! ../src/JPEGStream.h:42:8: warning: 'MakeCallback' is deprecated [-Wdeprecated-declarations]
npm ERR!   Nan::MakeCallback(Nan::GetCurrentContext()->Global(), (v8::Local<v8::Function>)dest->closure->fn, 2, argv);

Culprit seems to be old version (1.6.x) of canvas. This is only used in test.js so it should be straightforward to upgrade ... I will do so no.