chore(python): cosmetic improvements on generated code (#1554)

Add an explicit `-> None` type annotation to the `__init__` function of struct constructors, to be consistent with the recommendations of [PEP-484](https://www.python.org/dev/peps/pep-0484/#the-meaning-of-annotations).

Consistently put a space before the comma in `typing.Mapping[]` arguments. Previously the space was missing in the case of Maps, but present in the case of JSON objects.

The increased consistency of generated code would facilitate future refactorings, as less special cases have to be accounted for.

packages/jsii-pacmak/lib/targets/python.ts

@@ -756,7 +756,7 @@ class Struct extends BasePythonClassType {
 
     const constructorArguments = kwargs.length > 0 ? ['self', '*', ...kwargs] : ['self'];
 
-    code.openBlock(`def __init__(${constructorArguments.join(', ')})`);
+    code.openBlock(`def __init__(${constructorArguments.join(', ')}) -> None`);
     this.emitConstructorDocstring(code);
 
     // Re-type struct arguments that were passed as "dict"
@@ -1610,7 +1610,7 @@ class TypeResolver {
     const elementPythonType = this.toPythonType(ref.collection.elementtype, { optional: false });
     switch (ref.collection.kind) {
       case spec.CollectionKind.Array: return `typing.List[${elementPythonType}]`;
-      case spec.CollectionKind.Map: return `typing.Mapping[str,${elementPythonType}]`;
+      case spec.CollectionKind.Map: return `typing.Mapping[str, ${elementPythonType}]`;
       default:
         throw new Error(`Unsupported collection kind: ${ref.collection.kind}`);
     }

packages/jsii-pacmak/test/expected.jsii-calc-base-of-base/python/src/scope/jsii_calc_base_of_base/__init__.py

@@ -40,7 +40,7 @@ def hey(self) -> jsii.Number:
 
 @jsii.data_type(jsii_type="@scope/jsii-calc-base-of-base.VeryBaseProps", jsii_struct_bases=[], name_mapping={'foo': 'foo'})
 class VeryBaseProps():
-    def __init__(self, *, foo: "Very"):
+    def __init__(self, *, foo: "Very") -> None:
         """
         :param foo: -
         """

packages/jsii-pacmak/test/expected.jsii-calc-base/python/src/scope/jsii_calc_base/__init__.py

@@ -36,7 +36,7 @@ class _BaseProxy(Base):
 
 @jsii.data_type(jsii_type="@scope/jsii-calc-base.BaseProps", jsii_struct_bases=[scope.jsii_calc_base_of_base.VeryBaseProps], name_mapping={'foo': 'foo', 'bar': 'bar'})
 class BaseProps(scope.jsii_calc_base_of_base.VeryBaseProps):
-    def __init__(self, *, foo: scope.jsii_calc_base_of_base.Very, bar: str):
+    def __init__(self, *, foo: scope.jsii_calc_base_of_base.Very, bar: str) -> None:
         """
         :param foo: -
         :param bar: -

packages/jsii-pacmak/test/expected.jsii-calc-lib/python/src/scope/jsii_calc_lib/__init__.py

@@ -160,7 +160,7 @@ def baz(self) -> None:
 
 @jsii.data_type(jsii_type="@scope/jsii-calc-lib.MyFirstStruct", jsii_struct_bases=[], name_mapping={'anumber': 'anumber', 'astring': 'astring', 'first_optional': 'firstOptional'})
 class MyFirstStruct():
-    def __init__(self, *, anumber: jsii.Number, astring: str, first_optional: typing.Optional[typing.List[str]]=None):
+    def __init__(self, *, anumber: jsii.Number, astring: str, first_optional: typing.Optional[typing.List[str]]=None) -> None:
         """This is the first struct we have created in jsii.
 
         :param anumber: An awesome number value.
@@ -214,7 +214,7 @@ def __repr__(self) -> str:
 
 @jsii.data_type(jsii_type="@scope/jsii-calc-lib.StructWithOnlyOptionals", jsii_struct_bases=[], name_mapping={'optional1': 'optional1', 'optional2': 'optional2', 'optional3': 'optional3'})
 class StructWithOnlyOptionals():
-    def __init__(self, *, optional1: typing.Optional[str]=None, optional2: typing.Optional[jsii.Number]=None, optional3: typing.Optional[bool]=None):
+    def __init__(self, *, optional1: typing.Optional[str]=None, optional2: typing.Optional[jsii.Number]=None, optional3: typing.Optional[bool]=None) -> None:
         """This is a struct with only optional properties.
 
         :param optional1: The first optional!

packages/jsii-pacmak/test/expected.jsii-calc/python/src/jsii_calc/interface_in_namespace_includes_classes/__init__.py

@@ -39,7 +39,7 @@ def bar(self, value: typing.Optional[str]):
 
 @jsii.data_type(jsii_type="jsii-calc.InterfaceInNamespaceIncludesClasses.Hello", jsii_struct_bases=[], name_mapping={'foo': 'foo'})
 class Hello():
-    def __init__(self, *, foo: jsii.Number):
+    def __init__(self, *, foo: jsii.Number) -> None:
         """
         :param foo: 
 

packages/jsii-pacmak/test/expected.jsii-calc/python/src/jsii_calc/interface_in_namespace_only_interface/__init__.py

@@ -17,7 +17,7 @@
 
 @jsii.data_type(jsii_type="jsii-calc.InterfaceInNamespaceOnlyInterface.Hello", jsii_struct_bases=[], name_mapping={'foo': 'foo'})
 class Hello():
-    def __init__(self, *, foo: jsii.Number):
+    def __init__(self, *, foo: jsii.Number) -> None:
         """
         :param foo: 
 

packages/jsii-pacmak/test/expected.jsii-calc/python/src/jsii_calc/submodule/back_references/__init__.py

@@ -17,7 +17,7 @@
 
 @jsii.data_type(jsii_type="jsii-calc.submodule.back_references.MyClassReference", jsii_struct_bases=[], name_mapping={'reference': 'reference'})
 class MyClassReference():
-    def __init__(self, *, reference: jsii_calc.submodule.MyClass):
+    def __init__(self, *, reference: jsii_calc.submodule.MyClass) -> None:
         """
         :param reference: 
 

packages/jsii-pacmak/test/expected.jsii-calc/python/src/jsii_calc/submodule/child/__init__.py

@@ -55,7 +55,7 @@ class Goodness(enum.Enum):
 
 @jsii.data_type(jsii_type="jsii-calc.submodule.child.Structure", jsii_struct_bases=[], name_mapping={'bool': 'bool'})
 class Structure():
-    def __init__(self, *, bool: bool):
+    def __init__(self, *, bool: bool) -> None:
         """
         :param bool: