Test fail with pytest5 release due to syntax change #158

[   12s] =================================== FAILURES ===================================
[   12s] _____________ TestSchemaUnmarshal.test_string_format_invalid_value _____________
[   12s] 
[   12s] self = <test_schemas.TestSchemaUnmarshal object at 0x7fbd89b5e2e8>
[   12s] 
[   12s]     def test_string_format_invalid_value(self):
[   12s]         custom_format = 'custom'
[   12s]         schema = Schema('string', schema_format=custom_format)
[   12s]         value = 'x'
[   12s]     
[   12s]         with pytest.raises(
[   12s] >           InvalidSchemaValue, message='Failed to format value'
[   12s]         ):
[   12s] E       TypeError: Unexpected keyword arguments passed to pytest.raises: message
[   12s] E       Use context-manager form instead?
[   12s] 
[   12s] tests/unit/schema/test_schemas.py:171: TypeError
[   12s] =================== 1 failed, 288 passed, 1 xpassed in 0.91s ===================