Add support for Guzzle 7

[nmeirik]

Would you mind adding support for Guzzle 7 (to composer.json)? I'm attempting to install https://packagist.org/packages/microsoft/microsoft-graph#1.18.0 but it conflicts with the Guzzle version required by maknz/slack 1.7.0.

[alek13]

@nmeirik this package not supported.
But fork php-slack/slack supports Guzzle 7 in 1.12
This fork fully compatible with this package. You just need to change package name to https://packagist.org/packages/alek13/slack in your composer.json or simply run:

composer remove maknz/slack
composer require alek13/slack