Replace deprecated chunk module with inline IFF parser

The chunk module was deprecated in 3.11 and removed in 3.13.
Our usage was minimal (read name, size, data, skip padding),
so a small _read_iff_chunk() helper replaces it with no deps.

src/mudlib/zmachine/quetzal.py

@@ -12,14 +12,30 @@
 # root directory of this distribution.
 #
 
-# Woohoo!  Python has a module to parse IFF files, which is a generic
-# interchange format.  A Quetzal file is in fact a type of IFF file.
-import chunk
 import os
+import struct
 
 from . import bitfield, zstackmanager
 from .zlogging import log
 
+
+def _read_iff_chunk(f):
+    """Read one IFF chunk from a file-like object. Returns (name, data).
+
+    Raises EOFError when there are no more chunks to read.
+    """
+    header = f.read(8)
+    if len(header) < 8:
+        raise EOFError
+    name = header[:4]
+    (size,) = struct.unpack(">I", header[4:8])
+    data = f.read(size)
+    # IFF chunks are padded to even byte boundaries
+    if size % 2 == 1:
+        f.read(1)
+    return name, data
+
+
 # The general format of Queztal is that of a "FORM" IFF file, which is
 # a container class for 'chunks'.
 #
@@ -339,10 +355,8 @@ class QuetzalParser:
         log("Parsing chunks from byte data")
         try:
             while 1:
-                c = chunk.Chunk(self._file)
+                chunkname, chunk_data = _read_iff_chunk(self._file)
-                chunkname = c.getname()
+                chunksize = len(chunk_data)
-                chunksize = c.getsize()
-                chunk_data = c.read(chunksize)
                 log(f"** Found chunk ID {chunkname}: length {chunksize}")
                 self._last_loaded_metadata[chunkname] = chunksize
 
@@ -406,10 +420,8 @@ class QuetzalParser:
 
         try:
             while 1:
-                c = chunk.Chunk(self._file)
+                chunkname, data = _read_iff_chunk(self._file)
-                chunkname = c.getname()
+                chunksize = len(data)
-                chunksize = c.getsize()
-                data = c.read(chunksize)
                 log(f"** Found chunk ID {chunkname}: length {chunksize}")
                 self._last_loaded_metadata[chunkname] = chunksize
 

src/mudlib/zmachine/zstackmanager.py

@@ -104,6 +104,7 @@ class ZStackBottom:
 
     def __init__(self):
         self.program_counter = 0  # used as a cache only
+        self.return_addr = None
         self.stack = []
         self.local_vars = [0 for _ in range(15)]
 
@@ -201,15 +202,14 @@ class ZStackManager:
         current_routine = self._call_stack[-1]
 
         # Depending on many things, return stuff.
-        if exiting_routine.return_addr is not None:  # type: ignore[possibly-missing-attribute]
+        if exiting_routine.return_addr is not None:
-            if exiting_routine.return_addr == 0:  # type: ignore[possibly-missing-attribute]
+            if exiting_routine.return_addr == 0:
                 # Push to stack
                 self.push_stack(return_value)
-            elif 0 < exiting_routine.return_addr < 0x10:  # type: ignore[possibly-missing-attribute]
+            elif 0 < exiting_routine.return_addr < 0x10:
                 # Store in local var
-                self.set_local_variable(exiting_routine.return_addr - 1, return_value)  # type: ignore[possibly-missing-attribute]
+                self.set_local_variable(exiting_routine.return_addr - 1, return_value)
             else:
                 # Store in global var
-                self._memory.write_global(exiting_routine.return_addr, return_value)  # type: ignore[possibly-missing-attribute]
+                self._memory.write_global(exiting_routine.return_addr, return_value)
-
         return current_routine.program_counter