[dmypy] sort list of files for update by extension (#17245)

dmypy receives the list of updated files via `--update` flag. If this
list contains both `foo.py` and `foo.pyi`, the order matters. It seems
to process the first file in the list first. But if we have a `.pyi`
file, we want this to be processed first since this one contains the
typing information.
Let's reverse sort the list of updated files by the extension. This
should be a simple enough fix to resolve this.

Though there might be some edge cases where the list of files to update
contains just pyi files, but we might need to recheck the equivalent py
files even if not explicitly updated.

mypy/dmypy_server.py

@@ -383,6 +383,9 @@ def cmd_recheck(
             removals = set(remove)
             sources = [s for s in sources if s.path and s.path not in removals]
         if update:
+            # Sort list of file updates by extension, so *.pyi files are first.
+            update.sort(key=lambda f: os.path.splitext(f)[1], reverse=True)
+
             known = {s.path for s in sources if s.path}
             added = [p for p in update if p not in known]
             try: