Version 1.2.0-alpha.0

Contains the first few changes from #22:

• Redirecting disabled legacy SOAP API => new SOAP API
• Shipping web service
• Making Sabre / XML optional
• Remove guzzlehttp/promises dependency
• Gather all hot patches I have applied on production and merge them into this library
• HTTPlug for HTTP client + PSR-17 libs discovery
• Migrate to the Labeling web service v2.2 (Jan 25, 2019)
• Migrate to the Confirming web service v2.0 (Jan 25, 2019)

Important!
This update requires you to install a PSR-(1)7 implementation (e.g. nyholm/psr7) if you haven't already and a PSR-18 (async) HTTP client such as Guzzle or the Symfony HTTP Client March 2021 -- not anymore, the GuzzleClient is going to stay on the v1.x.x branch. To keep the SOAP API support, you will need to install sabre/xml version ~1.5.1. There are no plans to support a newer version of sabre/xml because the SOAP API is deprecated and because of the PHP 5.6-8.0 support it already provides.