Add __all__ in __init__.py

[kigawas]

Like this:

from coincurve.context import GLOBAL_CONTEXT, Context
from coincurve.keys import PrivateKey, PublicKey
from coincurve.utils import verify_signature

__all__ = [
    "GLOBAL_CONTEXT",
    "Context",
    "PrivateKey",
    "PublicKey",
    "verify_signature",
]