Adjust extended v3 HS address validation

Adjust extended v3 HS address validation

Mostly minor stylistic adjustments. The only functional bit is correcting a
unit test failure...

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	FAIL: test_identity_key_from_address
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	ValueError: '55555555555555555555555555555555555555555555555555555555.onion' isn't a valid hidden service v3 address

	During handling of the above exception, another exception occurred:

	Traceback (most recent call last):
  	File "/home/atagar/Desktop/stem/test/unit/descriptor/hidden_service_v3.py", line 220, in test_identity_key_from_address
    	self.assertRaisesWith(ValueError, 'Bad checksum (expected def7 but was 842e)', HiddenServiceDescriptorV3.identity_key_from_address, '5' * 56)
  	File "/home/atagar/Desktop/stem/stem/util/test_tools.py", line 249, in assertRaisesWith
    	return self.assertRaisesRegexp(exc_type, '^%s$' % re.escape(exc_msg), *args, **kwargs)
	AssertionError: "^Bad\ checksum\ \(expected\ def7\ but\ was\ 842e\)$" does not match "'55555555555555555555555555555555555555555555555555555555.onion' isn't a valid hidden service v3 address"

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