Drop dependency_overrides, bump CI script and SDK constraint (dart-lang/pubspec_parse#66)

diff --git a/pkgs/pubspec_parse/.github/workflows/test-package.yml b/pkgs/pubspec_parse/.github/workflows/test-package.yml
index d632645..df7aeb4 100644
--- a/pkgs/pubspec_parse/.github/workflows/test-package.yml
+++ b/pkgs/pubspec_parse/.github/workflows/test-package.yml
@@ -23,7 +23,7 @@
         sdk: [dev]
     steps:
       - uses: actions/checkout@v2
-      - uses: dart-lang/setup-dart@v0.3
+      - uses: dart-lang/setup-dart@v1.0
         with:
           sdk: ${{ matrix.sdk }}
       - id: install
@@ -46,11 +46,10 @@
       fail-fast: false
       matrix:
         os: [ubuntu-latest]
-        # change beta to 2.12.0 when released!
-        sdk: [beta, dev]
+        sdk: [2.12.0, dev]
     steps:
       - uses: actions/checkout@v2
-      - uses: dart-lang/setup-dart@v0.3
+      - uses: dart-lang/setup-dart@v1.0
         with:
           sdk: ${{ matrix.sdk }}
       - id: install
diff --git a/pkgs/pubspec_parse/CHANGELOG.md b/pkgs/pubspec_parse/CHANGELOG.md
index 97c410f..0c72a39 100644
--- a/pkgs/pubspec_parse/CHANGELOG.md
+++ b/pkgs/pubspec_parse/CHANGELOG.md
@@ -1,3 +1,5 @@
+## 1.0.1-dev
+
 ## 1.0.0
 
 - Migrate to null-safety.
diff --git a/pkgs/pubspec_parse/pubspec.yaml b/pkgs/pubspec_parse/pubspec.yaml
index 4179fea..bd9f678 100644
--- a/pkgs/pubspec_parse/pubspec.yaml
+++ b/pkgs/pubspec_parse/pubspec.yaml
@@ -2,11 +2,11 @@
 description: >-
   Simple package for parsing pubspec.yaml files with a type-safe API and rich
   error reporting.
-version: 1.0.0
+version: 1.0.1-dev
 repository: https://github.com/dart-lang/pubspec_parse
 
 environment:
-  sdk: '>=2.12.0-0 <3.0.0'
+  sdk: '>=2.12.0 <3.0.0'
 
 dependencies:
   checked_yaml: ^2.0.1
@@ -27,10 +27,3 @@
   test: ^1.0.0
   test_descriptor: ^2.0.0
   test_process: ^2.0.0
-
-dependency_overrides:
-  # Need to update dependencies on these packages
-  build_config: 0.4.6
-  build_runner: any
-  checked_yaml: ^2.0.1
-  json_annotation: 4.0.0