Formerly, if you wrote
SPI.frequency = 0
you would get the sightly erroneous error message
AttributeError: 'SPI' object has no attribute 'frequency'
In this case, a better message would read
AttributeError: 'SPI' object cannot assign attribute 'frequency'
This new message will both be used in the case where the attribute doesn't
exist at all (and the object has no dynamic attributes; most instances of
built in types behave this way), or if the attribute exists but is
read-only.
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