Clean up homogeneous composite sedes length pre-conditions

[hwwhww]

updated the issue: 2020, June 18th

What was wrong?

As ethereum/consensus-specs#1863 pointed out, it seems

py-ssz/ssz/sedes/basic.py

Lines 177 to 180 in 7e9c107

	if max_length < 1:
	raise ValueError(
	f"(Maximum) length of homogenous composites must be at least 1, got {max_length}"
	)

is only for Vector, and then override it in List. It made the illegal condition unclear.

How can it be fixed?

1. Use illegal condition max_length < 0 for HomogeneousProperCompositeSedes (wow it's so long)
2. Override vector types' (Vector and Bitvector) __init__() with illegal condition max_length < 1.

1. Remove HomogeneousProperCompositeSedes.__init_()
2. Update vector types' (Vector and Bitvector) __init__() with illegal condition max_length < 1.
3. Add list types' (List and Bitlist) __init__() with illegal condition max_length < 0.

[booleanfunction]

@hwwhww I think the __inti__()s for Bitvector and Vector have the override mentioned in 2. above? :)

py-ssz/ssz/sedes/bitvector.py

Lines 19 to 22 in 7e9c107

	class Bitvector(BitfieldCompositeSedes[BytesOrByteArray, bytes]):
	def __init__(self, bit_count: int) -> None:
	if bit_count <= 0:
	raise TypeError("Bit count cannot be zero or negative")

py-ssz/ssz/sedes/vector.py

Lines 25 to 32 in 7e9c107

	class Vector(
	HomogeneousProperCompositeSedes[
	Sequence[TSerializableElement], Tuple[TDeserializedElement, ...]
	]
	):
	def __init__(self, element_sedes: TSedes, length: int) -> None:
	if length <= 0:
	raise ValueError(f"Vectors must have a size of 1 or greater, got {length}")

[hwwhww]

@booleanfunction

You're right, sorry! Fixing the suggested proposal:

1. Replace HomogeneousProperCompositeSedes.__init_() with abstractmethod. Remove HomogeneousProperCompositeSedes.__init_()
2. Update vector types' (Vector and Bitvector) __init__() with illegal condition max_length < 1.
3. Add list types' (List and Bitlist) __init__() with illegal condition max_length < 0.

I think this is more clear than overriding?

[booleanfunction]

@hwwhww that seems like a good way to do it :)