Previously the TxInput was constructed with a dummy scriptsig (in Transaction.txinputs()). Now it is constructed via TxInput.from_keys, which makes these values mandatory.

scriptsig is now an optional argument and the sequence number is a mandatory positional argument in the TxInput.__init\\ constructor, so I had to change the order of the parameters to have the mandatory argument before the optional arguments.

I thought it would be best to have the arguments in the same order for TxInput.from_scriptsig than in the constructor, even though in from_scriptsig the scriptsig is also mandatory. It is just for consistency.

Previously the TxInput class was accepting garbage as a scriptsig, now it requires a well formed script, which was already required by the legacy code when deserializing an input.
This test was added very recently (last week) and never used the legacy deserialization code, that's why I got away with using garbage data as a scriptsig.

I'm not actually sure about this. Some type analysis tools seem to assume that the attribute is of the same type as what is passed to him from the input argument, but that is not necessarily always correct. For example here the type are not exactly the same:

class Tx:
    def __init__(self, inputs: Optional[List] =: None, outputs: Optional[List] =None) -> List:
         self.inputs: List = inputs if inputs is not None else []
         self.outputs: List = outputs if outputs is not None else []

I think it is best to specify the type on both to avoid any ambiguity.
I will add the typehint to scriptsig as well.

del would always raise an error if scriptsig is not specified. It only deletes the item from the dict.
pop also returns the item before deleting it from the dict (unused here), and allows specifying a default value to return in case the item is not found. If a default value is specified, it will not raise an error if the item is not in the dict.

So this changes fixes an error if d does not have scriptsig. Previously this was never possible, as the scriptsig was mandatory in __init__. Now it is possible to reach this line without having a scriptsig in the dict, if the input was created with from_keys.

Also previously scriptsig never changed after __init__, whereas the other attributes (_pubkeys...) could be initially None, then assigned later when parsing the scriptsig.
I anticipate that scriptsig will be computed on demand and cached for complete (fully signed) inputs in a subsequent diff, when I move the TxInput serialization code to the class.