bump to 3.3.1

CHANGELOG.md

+## 3.3.1
+* Fix #354
+
 ## 3.3.0
 * Null-safety support
 * Fix #350

pubspec.yaml

 name: fluwx
 description: The capability of implementing WeChat SDKs in Flutter. With Fluwx, developers can use WeChatSDK easily, such as sharing, payment, lanuch mini program and etc.
-version: 3.3.0
+version: 3.3.1
 homepage: https://github.com/JarvanMo/fluwx
 
 environment:
-  sdk: ">=2.12.0-0 <3.0.0"
+  sdk: ">=2.12.0 <3.0.0"
   flutter: ">=1.12.0"
 
 dependencies: