There seem to be some version dependency issue with that email-validator.
I solved it by updating the composer.json to the newer version before updating Drupal:
composer require drupal/core:~8.7.0 --no-update
composer require webflo/drupal-core-require-dev:~8.7.0 --no-update
composer update drupal/core webflo/drupal-core-require-dev --with-dependencies --dry-run
composer update drupal/core webflo/drupal-core-require-dev --with-dependencies
YMMV, here is the Issues where you can get more help if this doesn't work (check the related Issues as well):
Cannot update to Drupal 8.7.0 from 8.6.15 -- egulias/email-validator version conflict
Don't forget to attach your composer.json and composer.lock there so the problem can be properly diagnosed.