[sdk/python] Improve ResourceOptions.merge type (#4484)

* [sdk/python] Improve `ResoruceOptions.merge` type

The implementation correctly handles `None` inputs, so the type should allow these as well.

* Add CHANGELOG

CHANGELOG.md

@@ -16,10 +16,12 @@ CHANGELOG
 - Ensure Go accessor methods correctly support nested fields of optional outputs
   [#4456](https://github.com/pulumi/pulumi/pull/4456)
 
+- Improve `ResourceOptions.merge` type in Python SDK
+  [#4484](https://github.com/pulumi/pulumi/pull/4484)
+
 - Ensure generated Python module names are keyword-safe.
   [#4473](https://github.com/pulumi/pulumi/pull/4473)
 
-
 ## 2.0.0 (2020-04-16)
 
 - CLI behavior change.  Commands in non-interactive mode (i.e. when `pulumi` has its output piped to

sdk/python/lib/pulumi/resource.py

@@ -444,7 +444,7 @@ class ResourceOptions:
 
     # pylint: disable=method-hidden
     @staticmethod
-    def merge(opts1: 'ResourceOptions', opts2: 'ResourceOptions') -> 'ResourceOptions':
+    def merge(opts1: Optional['ResourceOptions'], opts2: Optional['ResourceOptions']) -> 'ResourceOptions':
         """
         merge produces a new ResourceOptions object with the respective attributes of the `opts1`
         instance in it with the attributes of `opts2` merged over them.