Strictly enforce instance attrs in critical functional test classes.

Additionally, removed redundant parentheses and added PEP-8 compliant
spacing around those classes.

test/functional/test_framework/script.py

@@ -26,7 +26,7 @@ def hash160(s):
 _opcode_instances = []
 class CScriptOp(int):
     """A single script opcode"""
-    __slots__ = []
+    __slots__ = ()
 
     @staticmethod
     def encode_op_pushdata(d):
@@ -361,8 +361,11 @@ class CScriptTruncatedPushDataError(CScriptInvalidError):
         self.data = data
         super(CScriptTruncatedPushDataError, self).__init__(msg)
 
+
 # This is used, eg, for blockchain heights in coinbase scripts (bip34)
-class CScriptNum():
+class CScriptNum:
+    __slots__ = ("value",)
+
     def __init__(self, d=0):
         self.value = d
 
@@ -393,6 +396,8 @@ class CScript(bytes):
 
     iter(script) however does iterate by opcode.
     """
+    __slots__ = ()
+
     @classmethod
     def __coerce_instance(cls, other):
         # Coerce other into bytes