v_41: Add an error if period exceeds the limit

This release adds a new error message to Key.activate that will be thrown if the period exceeds the permitted limit in Python. The problem is described in https://github.com/Cryptolens/cryptolens-python/tree/master#the-expiration-date-cannot-be-converted-to-a-datetime-object-please-try-setting-the-period-to-a-lower-value, but in essence, the issue is that the timestamp representation of the expiration date causes an overflow in Python, and thus an error is thrown.