Add support for PHP 8.0+

[viktorbijlenga]

Could we please get an update for the getanewsletter/api-php package to make it work better with PHP 8.0+ and above?

The current version of the getanewsletter/api-php package relies on nategood/httpful, and it doesn't play nice with PHP 8.0 at the moment.

[viktorbijlenga]

nategood/httpful now supports PHP 8+ and includes an important security patch.