Add BLSPubkey and BLSSignature

[ChihChengLiang]

What was wrong?

These are two types required by BLS module.

How can it be fixed?

# eth_typing/bls.py
BLSPubkey = NewType('BLSPubkey', bytes)  # bytes48
BLSSignature = NewType('BLSSignature', bytes)  # bytes96

# eth_typing/__init__.py
from .bls import (
    BLSPubkey,
    BLSSignature,
)

[ChihChengLiang]

Fixed in #13