Add support for mailgun-php ^3.0

[DieterHolvoet]

This PR updates the mailgun/mailgun-php package to ^3.0, which fixes dependency conflicts with Symfony 4.2+ (#78, #79), as well as php-http/guzzle7-adapter. It continues work on #84.

[amolochitis]

I get these errors when trying to use the Mailgun Library too.

[20-Nov-2020 10:56:33 America/Chicago] PHP Fatal error: Uncaught Http\Discovery\Exception\DiscoveryFailedException: Could not find resource using any discovery strategy. Find more information at http://docs.php-http.org/en/latest/discovery.html#common-errors

• Puli Factory is not available
• No valid candidate found using strategy "Http\Discovery\Strategy\CommonClassesStrategy". We tested the following candidates: .
• No valid candidate found using strategy "Http\Discovery\Strategy\CommonPsr17ClassesStrategy". We tested the following candidates: Phalcon\Http\Message\UriFactory, Nyholm\Psr7\Factory\Psr17Factory, Zend\Diactoros\UriFactory, GuzzleHttp\Psr7\HttpFactory, Http\Factory\Diactoros\UriFactory, Http\Factory\Guzzle\UriFactory, Http\Factory\Slim\UriFactory, Laminas\Diactoros\UriFactory.

Is this a solution to the error?

[DieterHolvoet]

@amolochitis that error means you don't have a PSR-7 implementation and HTTP client installed. I changed the suggestion in the README to nyholm/psr7 and kriswallsmith/buzz. These are the ones suggested in the mailgun/mailgun-php and they are working fine for me.

[boboldehampsink]

Is this package still alive?

[davola]

As of today (2021-08-25) I can confirm this PR for supporting mailgun v3 works just fine 👌 Thanks @DieterHolvoet 🙏 !

[karensg]

Just released a new version with #88.
Should have all our problems solved. Let me know whether there any issues with it.