Add bool for permanent delete

[slominskir]

The SchemaRegistryClient delete subject function doesn't currently allow the boolean permanent flag:

confluent-kafka-python/src/confluent_kafka/schema_registry/schema_registry_client.py

Lines 441 to 461 in ca09a63

	def delete_subject(self, subject_name):
	"""
	Deletes the specified subject and its associated compatibility level if
	registered. It is recommended to use this API only when a topic needs
	to be recycled or in development environments.
	Args:
	subject_name (str): subject name
	Returns:
	list(int): Versions deleted under this subject
	Raises:
	SchemaRegistryError: if the request was unsuccessful.
	See Also:
	`DELETE Subject API Reference <https://docs.confluent.io/current/schema-registry/develop/api.html#delete--subjects-(string-%20subject)>`_
	""" # noqa: E501
	return self._rest_client.delete('subjects/{}'
	.format(_urlencode(subject_name)))

Soft vs Hard delete is included in the API docs though:
DELETE REST API docs

[ghost]

@confluentinc It looks like @slominskir just signed our Contributor License Agreement. 👍

Always at your service,

clabot

[mhowlett]

thanks @slominskir - LGTM

[mhowlett]

oh, flake8 is complaining about line 450, a blank line with spaces on it - can you fix that and check the travis logs for any additional issues related to this PR? sorry about that and thanks

[slominskir]

I removed the white space flake8 had flagged, but now flake8 is logging a failure in a file that I never touched:

$ flake8
./src/confluent_kafka/avro/load.py:18:1: F401 'sys' imported but unused
The command "flake8" exited with 1.

Looks like issue is due to this commit merged before I created a patch branch: 67b0052#diff-118623b097e0145cb52700e9c99901e9cf79616a8936a562f42763aa9ad4f4ec

[mhowlett]

ok, that was my fault :-), sorry about that. thanks for the additional PR :-).