Bump SDK constraints for pub (#19)

Use a 2.12.0 lower bound since pub does not understand allowed
experiments for earlier versions.

Use a 3.0.0 upper bound to avoid a warning in pub and to give some
flexibility in publishing for stable.
diff --git a/CHANGELOG.md b/CHANGELOG.md
index bfc25b6..afa0580 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,3 +1,8 @@
+## 1.2.0-nullsafety.3
+
+* Update SDK constraints to `>=2.12.0-0 <3.0.0` based on beta release
+  guidelines.
+
 ## 1.2.0-nullsafety.2
 
 * Allow prerelease versions of the 2.12 sdk.
diff --git a/pubspec.yaml b/pubspec.yaml
index a1dc00e..751bcc5 100644
--- a/pubspec.yaml
+++ b/pubspec.yaml
@@ -1,11 +1,10 @@
 name: charcode
-version: 1.2.0-nullsafety.2
+version: 1.2.0-nullsafety.3
 description: >
   Constants for ASCII and common non-ASCII character codes.
 
 environment:
-  # This must remain a tight constraint until nnbd is stable
-  sdk: '>=2.10.0-0 <2.12.0'
+  sdk: ">=2.12.0-0 <3.0.0"
 
 dev_dependencies:
   test: ^1.16.0-nullsafety