fix(cli): fix Python sample-app template

packages/aws-cdk/lib/init-templates/sample-app/python/app.py

@@ -1,12 +1,12 @@
 #!/usr/bin/env python3
 
-from aws_cdk import cdk
+from aws_cdk import core
 
 from hello.hello_stack import MyStack
 
 
-app = cdk.App()
+app = core.App()
 MyStack(app, "hello-cdk-1", env={'region': 'us-east-2'})
 MyStack(app, "hello-cdk-2", env={'region': 'us-west-2'})
 
-app.run()
+app.synth()

packages/aws-cdk/lib/init-templates/sample-app/python/hello/hello_construct.py

@@ -1,17 +1,17 @@
 from aws_cdk import (
     aws_iam as iam,
     aws_s3 as s3,
-    cdk,
+    core,
 )
 
 
-class HelloConstruct(cdk.Construct):
+class HelloConstruct(core.Construct):
 
     @property
     def buckets(self):
         return tuple(self._buckets)
 
-    def __init__(self, scope: cdk.Construct, id: str, num_buckets: int) -> None:
+    def __init__(self, scope: core.Construct, id: str, num_buckets: int) -> None:
         super().__init__(scope, id)
         self._buckets = []
         for i in range(0, num_buckets):

packages/aws-cdk/lib/init-templates/sample-app/python/hello/hello_stack.py

@@ -3,20 +3,20 @@
     aws_sqs as sqs,
     aws_sns as sns,
     aws_sns_subscriptions as subs,
-    cdk
+    core
 )
 
 from hello_construct import HelloConstruct
 
 
-class MyStack(cdk.Stack):
+class MyStack(core.Stack):
 
-    def __init__(self, app: cdk.App, id: str, **kwargs) -> None:
+    def __init__(self, app: core.App, id: str, **kwargs) -> None:
         super().__init__(app, id, **kwargs)
 
         queue = sqs.Queue(
             self, "MyFirstQueue",
-            visibility_timeout_sec=300,
+            visibility_timeout=core.Duration.seconds(300),
         )
 
         topic = sns.Topic(

packages/aws-cdk/lib/init-templates/sample-app/python/setup.py

@@ -19,7 +19,7 @@
     packages=setuptools.find_packages(where="hello"),
 
     install_requires=[
-        "aws-cdk.cdk",
+        "aws-cdk.core",
         "aws-cdk.aws_iam",
         "aws-cdk.aws_sqs",
         "aws-cdk.aws_sns",

packages/aws-cdk/lib/init-templates/sample-app/python/tests/unit/test_hello_construct.py

@@ -1,14 +1,14 @@
 import unittest
 
-from aws_cdk import cdk
+from aws_cdk import core
 
 from hello.hello_construct import HelloConstruct
 
 class TestHelloConstruct(unittest.TestCase):
 
     def setUp(self):
-        self.app = cdk.App()
-        self.stack = cdk.Stack(self.app, "TestStack")
+        self.app = core.App()
+        self.stack = core.Stack(self.app, "TestStack")
 
     def test_num_buckets(self):
         num_buckets = 10