Tag the version as 3.0.0-alpha

Tag the version as 3.0.0-alpha as we aren't done with changes that we want done with 3.0.0
diff --git a/pubspec.yaml b/pubspec.yaml
index 839a096..afad5bf 100644
--- a/pubspec.yaml
+++ b/pubspec.yaml
@@ -1,5 +1,5 @@
 name: mockito
-version: 3.0.0
+version: 3.0.0-alpha
 authors:
   - Dmitriy Fibulwinter <fibulwinter@gmail.com>
   - Dart Team <misc@dartlang.org>