bpo-35312: Make lib2to3.pgen2.parse.ParseError round-trip pickle-able. (GH-10710)

diff --git a/Lib/lib2to3/pgen2/parse.py b/Lib/lib2to3/pgen2/parse.py
index 6bebdbb..cf3fcf7 100644
--- a/Lib/lib2to3/pgen2/parse.py
+++ b/Lib/lib2to3/pgen2/parse.py
@@ -24,6 +24,9 @@
         self.value = value
         self.context = context
 
+    def __reduce__(self):
+        return type(self), (self.msg, self.type, self.value, self.context)
+
 class Parser(object):
     """Parser engine.