Update environment dependency

pubspec.yaml

@@ -1,11 +1,11 @@
 name: event_bus
-version: 2.0.0
+version: 2.0.1
 author: Marco Jakob <[email protected]>
 description: A simple Event Bus using Dart Streams for decoupling applications
 repository: https://github.com/marcojakob/dart-event-bus
 homepage: https://code.makery.ch/library/dart-event-bus/
 environment:
-  sdk: '>=2.12.0 <3.0.0'
+  sdk: '>=2.12.0 <4.0.0'
 dev_dependencies:
   build_runner: ^1.2.5
   build_web_compilers: ^2.16.4