vaadin-router doesn't run in npm7 (node 15)

[haijian-vaadin]

To reproduce:

1. Install npm 7
2. in the vaadin-router root directory, run npm install
3. See the error messages

vaadin-router % npm install
npm ERR! code ERESOLVE
npm ERR! ERESOLVE unable to resolve dependency tree
npm ERR! 
npm ERR! Found: eslint@7.18.0
npm ERR! node_modules/eslint
npm ERR!   dev eslint@"^7.15.0" from the root project
npm ERR! 
npm ERR! Could not resolve dependency:
npm ERR! dev eslint-config-vaadin@"0.2.7" from the root project
npm ERR! 
npm ERR! Conflicting peer dependency: eslint@5.16.0
npm ERR! node_modules/eslint
npm ERR!   peer eslint@"^5.0.0" from eslint-config-vaadin@0.2.7
npm ERR!   node_modules/eslint-config-vaadin
npm ERR! 
npm ERR! Fix the upstream dependency conflict, or retry
npm ERR! this command with --force, or --legacy-peer-deps
npm ERR! to accept an incorrect (and potentially broken) dependency resolution.
npm ERR! 
npm ERR! See /Users/haijian/.npm/eresolve-report.txt for a full report.

npm ERR! A complete log of this run can be found in:
npm ERR!     /Users/haijian/.npm/_logs/2021-01-29T10_26_31_733Z-debug.log

[Haprog]

Currently it works with npm 7 if you run:

npm install --legacy-peer-deps

Latest Yarn also works:

yarn install