BIP352: complete type annotations on bitcoin_utils class methods

Direct follow-up to PR #2154 (which annotated the free-function half
of bip-0352/bitcoin_utils.py) and 2f7117c ("BIP352: fix Any typing").
The four classes in this file — COutPoint, VinInfo, CScriptWitness,
and CTxInWitness — still had unannotated `__init__`, `serialize`,
`deserialize`, and `is_null` methods. mypy could not flow types
through the surrounding reference.py code that constructs and passes
these objects.

Annotate every method on all four classes:

- COutPoint:    __init__ (hash, n), serialize -> bytes, deserialize -> None
- VinInfo:      __init__ (typed Optional defaults for the three
                construct-on-None fields)
- CScriptWitness: __init__ -> None, is_null -> bool, plus an inline
                stack: List[bytes] declaration matching what the
                rest of the file already assumes
- CTxInWitness: __init__ -> None, deserialize -> "CTxInWitness"
                (forward ref since it returns self), is_null -> bool

Adds Optional to the existing typing import (List was already added
by #2154). No behavior changes.

Verified: mypy --ignore-missing-imports ./bip-0352/bitcoin_utils.py
reports "Success: no issues found in 1 source file"; round-trip
smoke tests on COutPoint serialize/deserialize, CScriptWitness
is_null with empty + non-empty stack, CTxInWitness deserialize
returning self, and VinInfo default-construction all match the
pre-change behavior.

bip-0352/bitcoin_utils.py

@@ -3,7 +3,7 @@ import struct
 from io import BytesIO
 from ripemd160 import ripemd160
 from secp256k1lab.secp256k1 import Scalar
-from typing import List, Union
+from typing import List, Optional, Union
 
 
 def from_hex(hex_string: str) -> BytesIO:
@@ -65,17 +65,17 @@ def deser_string_vector(f: BytesIO) -> List[bytes]:
 class COutPoint:
     __slots__ = ("hash", "n",)
 
-    def __init__(self, hash=b"", n=0,):
+    def __init__(self, hash: bytes = b"", n: int = 0) -> None:
         self.hash = hash
         self.n = n
 
-    def serialize(self):
+    def serialize(self) -> bytes:
         r = b""
         r += self.hash
         r += struct.pack("<I", self.n)
         return r
 
-    def deserialize(self, f):
+    def deserialize(self, f: BytesIO) -> None:
         self.hash = f.read(32)
         self.n = struct.unpack("<I", f.read(4))[0]
 
@@ -83,7 +83,14 @@ class COutPoint:
 class VinInfo:
     __slots__ = ("outpoint", "scriptSig", "txinwitness", "prevout", "private_key")
 
-    def __init__(self, outpoint=None, scriptSig=b"", txinwitness=None, prevout=b"", private_key=None):
+    def __init__(
+        self,
+        outpoint: Optional["COutPoint"] = None,
+        scriptSig: bytes = b"",
+        txinwitness: Optional["CTxInWitness"] = None,
+        prevout: bytes = b"",
+        private_key: Optional[Scalar] = None,
+    ) -> None:
         if outpoint is None:
             self.outpoint = COutPoint()
         else:
@@ -103,11 +110,11 @@ class VinInfo:
 class CScriptWitness:
     __slots__ = ("stack",)
 
-    def __init__(self):
+    def __init__(self) -> None:
         # stack is a vector of strings
-        self.stack = []
+        self.stack: List[bytes] = []
 
-    def is_null(self):
+    def is_null(self) -> bool:
         if self.stack:
             return False
         return True
@@ -116,14 +123,14 @@ class CScriptWitness:
 class CTxInWitness:
     __slots__ = ("scriptWitness",)
 
-    def __init__(self):
+    def __init__(self) -> None:
         self.scriptWitness = CScriptWitness()
 
-    def deserialize(self, f: BytesIO):
+    def deserialize(self, f: BytesIO) -> "CTxInWitness":
         self.scriptWitness.stack = deser_string_vector(f)
         return self
 
-    def is_null(self):
+    def is_null(self) -> bool:
         return self.scriptWitness.is_null()