Apply suggestions from code review

Co-authored-by: lpascal-ledger <[email protected]>

test_utils/musig2.py

@@ -288,7 +288,7 @@ def __init__(self, key: KeyPlaceholder, tree=Optional[Tree]):
         self.tree: Optional[Tree] = tree
 
     @classmethod
-    def from_string(cls, input_string):
+    def from_string(cls, input_string: str) -> "TrDescriptorTemplate":
         parser = cls.Parser(input_string.replace("/**", "/<0;1>/*"))
         return parser.parse()
 
@@ -298,7 +298,7 @@ def __init__(self, input):
             self.index = 0
             self.length = len(input)
 
-        def parse(self):
+        def parse(self) -> "TrDescriptorTemplate":
             if self.input.startswith('tr('):
                 self.consume('tr(')
                 key = self.parse_keyplaceholder()
@@ -312,7 +312,7 @@ def parse(self):
                 raise Exception(
                     "Syntax error: Input does not start with 'tr('")
 
-        def parse_keyplaceholder(self):
+        def parse_keyplaceholder(self) -> KeyPlaceholder:
             if self.peek() == '@':
                 self.consume('@')
                 key_index = self.parse_num()