python: several typing and style improvements (#16378)

* python: several typing and style improvements

The generated (python) code fails with several standard validation tools,
including `flake8`, `mypy`, and `autoflake`. While fixing every possible
violation -- especially wrto typing -- woudl be a project, some of the
changes are fairly easy.

 - The `autoflake` tool picks up on unused imports. These can just be removed.

 - The `mypy` tool picks up on numerious typing violations, especially if set
   to its strictest mode. As a starting point, all functions ought to annotate
   a return type, including constructors, even if the return type is `None`
   because otherwise the functions are omitted from type checking and it's
   impossible to make incremental progress towards adding types.

 - The `flake8` tool mostly finds whitespace and line-length issues; while
   line-length standards very, the source already includes several flake8
   ignores, so it seems safe to add a few more.

* Add generated files

* Restore imports used by `AbstractPythonCodegen.java`

* Update generated files

samples/client/echo_api/python/openapi_client/exceptions.py

@@ -19,7 +19,7 @@ class OpenApiException(Exception):
 
 class ApiTypeError(OpenApiException, TypeError):
     def __init__(self, msg, path_to_item=None, valid_classes=None,
-                 key_type=None):
+                 key_type=None) -> None:
         """ Raises an exception for TypeErrors
 
         Args:
@@ -47,7 +47,7 @@ class ApiTypeError(OpenApiException, TypeError):
 
 
 class ApiValueError(OpenApiException, ValueError):
-    def __init__(self, msg, path_to_item=None):
+    def __init__(self, msg, path_to_item=None) -> None:
         """
         Args:
             msg (str): the exception message
@@ -65,7 +65,7 @@ class ApiValueError(OpenApiException, ValueError):
 
 
 class ApiAttributeError(OpenApiException, AttributeError):
-    def __init__(self, msg, path_to_item=None):
+    def __init__(self, msg, path_to_item=None) -> None:
         """
         Raised when an attribute reference or assignment fails.
 
@@ -84,7 +84,7 @@ class ApiAttributeError(OpenApiException, AttributeError):
 
 
 class ApiKeyError(OpenApiException, KeyError):
-    def __init__(self, msg, path_to_item=None):
+    def __init__(self, msg, path_to_item=None) -> None:
         """
         Args:
             msg (str): the exception message
@@ -102,7 +102,7 @@ class ApiKeyError(OpenApiException, KeyError):
 
 class ApiException(OpenApiException):
 
-    def __init__(self, status=None, reason=None, http_resp=None):
+    def __init__(self, status=None, reason=None, http_resp=None) -> None:
         if http_resp:
             self.status = http_resp.status
             self.reason = http_resp.reason
@@ -129,30 +129,30 @@ class ApiException(OpenApiException):
 
 class BadRequestException(ApiException):
 
-    def __init__(self, status=None, reason=None, http_resp=None):
+    def __init__(self, status=None, reason=None, http_resp=None) -> None:
         super(BadRequestException, self).__init__(status, reason, http_resp)
 
 class NotFoundException(ApiException):
 
-    def __init__(self, status=None, reason=None, http_resp=None):
+    def __init__(self, status=None, reason=None, http_resp=None) -> None:
         super(NotFoundException, self).__init__(status, reason, http_resp)
 
 
 class UnauthorizedException(ApiException):
 
-    def __init__(self, status=None, reason=None, http_resp=None):
+    def __init__(self, status=None, reason=None, http_resp=None) -> None:
         super(UnauthorizedException, self).__init__(status, reason, http_resp)
 
 
 class ForbiddenException(ApiException):
 
-    def __init__(self, status=None, reason=None, http_resp=None):
+    def __init__(self, status=None, reason=None, http_resp=None) -> None:
         super(ForbiddenException, self).__init__(status, reason, http_resp)
 
 
 class ServiceException(ApiException):
 
-    def __init__(self, status=None, reason=None, http_resp=None):
+    def __init__(self, status=None, reason=None, http_resp=None) -> None:
         super(ServiceException, self).__init__(status, reason, http_resp)