v0.9.0

• Added PHP 8 support
• Added the ability to prepend middleware via Client::withPrependedMiddleware() (see #69)
• The signature of MiddlewareHandler::create() changed to:

  - public static function create(Handler $handler, Middleware $middleware, Middleware ...$middlewares) : Handler
  + public static function create(Handler $handler, array $middleware, bool $prepend = false) : Handler

• Added the ClientException interface, all client exception classes now implement this interface
• Enhanced RetryMiddleware:
  • Added protection against infinite retries (the maximum number of retries allowed is 10)
  • Only сlient exceptions (implementing the ClientException interface) can be retried
  • Tuned default parameters for exponential retries (changed baseMs from 100 to 10)
  • Limit the maximum retry interval to 1 min
  • Added equal jitter to retry intervals
• Reverted ee6bc94 (see 8a8765b)
• QA:
  • Added the PhpUnitCompat trait
  • Added SpyMiddleware
  • Switched to travis-ci.com
  • Added Scrutinizer configuration