fix(lambda-python): runtime is now required (aws#18143)

BREAKING CHANGE: Runtime is now required for `LambdaPython`

fixes aws#10248

----

*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

packages/@aws-cdk/aws-lambda-python/README.md

@@ -27,9 +27,9 @@ Define a `PythonFunction`:
 ```ts
 new lambda.PythonFunction(this, 'MyFunction', {
   entry: '/path/to/my/function', // required
+  runtime: Runtime.PYTHON_3_6, // required
   index: 'my_index.py', // optional, defaults to 'index.py'
   handler: 'my_exported_func', // optional, defaults to 'handler'
-  runtime: Runtime.PYTHON_3_6, // optional, defaults to lambda.Runtime.PYTHON_3_7
 });
 ```
 
@@ -86,6 +86,7 @@ layer.
 ```ts
 new lambda.PythonFunction(this, 'MyFunction', {
   entry: '/path/to/my/function',
+  runtime: Runtime.PYTHON_3_6,
   layers: [
     new lambda.PythonLayerVersion(this, 'MyLayer', {
       entry: '/path/to/my/layer', // point this to your library's directory

packages/@aws-cdk/aws-lambda-python/lib/function.ts

@@ -34,10 +34,8 @@ export interface PythonFunctionProps extends lambda.FunctionOptions {
   /**
    * The runtime environment. Only runtimes of the Python family are
    * supported.
-   *
-   * @default lambda.Runtime.PYTHON_3_7
    */
-  readonly runtime?: lambda.Runtime;
+  readonly runtime: lambda.Runtime;
 
   /**
    * Determines how asset hash is calculated. Assets will get rebuild and

packages/@aws-cdk/aws-lambda-python/test/function.test.ts

@@ -45,6 +45,7 @@ beforeEach(() => {
 test('PythonFunction with defaults', () => {
   new PythonFunction(stack, 'handler', {
     entry: 'test/lambda-handler',
+    runtime: Runtime.PYTHON_3_8,
   });
 
   expect(bundle).toHaveBeenCalledWith(expect.objectContaining({
@@ -62,6 +63,7 @@ test('PythonFunction with index in a subdirectory', () => {
     entry: 'test/lambda-handler-sub',
     index: 'inner/custom_index.py',
     handler: 'custom_handler',
+    runtime: Runtime.PYTHON_3_8,
   });
 
   expect(bundle).toHaveBeenCalledWith(expect.objectContaining({
@@ -78,12 +80,14 @@ test('throws when index is not py', () => {
   expect(() => new PythonFunction(stack, 'Fn', {
     entry: 'test/lambda-handler',
     index: 'index.js',
+    runtime: Runtime.PYTHON_3_8,
   })).toThrow(/Only Python \(\.py\) index files are supported/);
 });
 
 test('throws when entry does not exist', () => {
   expect(() => new PythonFunction(stack, 'Fn', {
     entry: 'notfound',
+    runtime: Runtime.PYTHON_3_8,
   })).toThrow(/Cannot find index file at/);
 });
 
@@ -99,27 +103,31 @@ test('allows specifying hash type', () => {
     entry: 'test/lambda-handler-nodeps',
     index: 'index.py',
     handler: 'handler',
+    runtime: Runtime.PYTHON_3_8,
   });
 
   new PythonFunction(stack, 'source2', {
     entry: 'test/lambda-handler-nodeps',
     index: 'index.py',
     handler: 'handler',
     assetHashType: AssetHashType.SOURCE,
+    runtime: Runtime.PYTHON_3_8,
   });
 
   new PythonFunction(stack, 'output', {
     entry: 'test/lambda-handler-nodeps',
     index: 'index.py',
     handler: 'handler',
     assetHashType: AssetHashType.OUTPUT,
+    runtime: Runtime.PYTHON_3_8,
   });
 
   new PythonFunction(stack, 'custom', {
     entry: 'test/lambda-handler-nodeps',
     index: 'index.py',
     handler: 'handler',
     assetHash: 'MY_CUSTOM_HASH',
+    runtime: Runtime.PYTHON_3_8,
   });
 
   Template.fromStack(stack).hasResourceProperties('AWS::Lambda::Function', {