perf: drop the initial stack copy (#2670)

Co-authored-by: Richard Si <63936253+ichard26@users.noreply.github.com>

CHANGES.md

@@ -26,6 +26,8 @@
 - Fix styling of starred expressions inside `match` subject (#2667)
 - Fix parser error location on invalid syntax in a `match` statement (#2649)
 - Fix Python 3.10 support on platforms without ProcessPoolExecutor (#2631)
+- Improve parsing performance on code that uses `match` under `--target-version py310`
+  up to ~50% (#2670)
 
 ### Packaging
 

src/blib2to3/pgen2/parse.py

@@ -53,7 +53,7 @@ def __init__(self, parser: "Parser", ilabels: List[int], context: Context) -> No
         self.context = context  # not really matter
 
         self._dead_ilabels: Set[int] = set()
-        self._start_point = copy.deepcopy(self.parser.stack)
+        self._start_point = self.parser.stack
         self._points = {ilabel: copy.deepcopy(self._start_point) for ilabel in ilabels}
 
     @property