Export a private key in PEM-format

		:param password: If it is not None, then result will be encrypt with given password
		:return: bytes

def export_private_key(self, password=None):
		""" Export a private key in PEM-format

		:param password: If it is not None, then result will be encrypt with given password
		:return: bytes
		"""

		if self.__private_key is None:
			raise ValueError('Unable to call this method. Private key must be set')

		if password is not None:
			if isinstance(password, str) is True:
				password = password.encode()
			return self.__private_key.private_bytes(
				encoding=serialization.Encoding.PEM,
				format=serialization.PrivateFormat.PKCS8,
				encryption_algorithm=serialization.BestAvailableEncryption(password)
			)
		return self.__private_key.private_bytes(
			encoding=serialization.Encoding.PEM,
			format=serialization.PrivateFormat.TraditionalOpenSSL,
			encryption_algorithm=serialization.NoEncryption()
		)