argparse: Use Literal for action & nargs add_argument parameter types (#7329)

stdlib/argparse.pyi

@@ -1,5 +1,20 @@
 import sys
-from typing import IO, Any, Callable, Generator, Generic, Iterable, NoReturn, Pattern, Protocol, Sequence, TypeVar, overload
+from typing import (
+    IO,
+    Any,
+    Callable,
+    Generator,
+    Generic,
+    Iterable,
+    NewType,
+    NoReturn,
+    Pattern,
+    Protocol,
+    Sequence,
+    TypeVar,
+    overload,
+)
+from typing_extensions import Literal
 
 if sys.version_info >= (3, 9):
     __all__ = [
@@ -48,12 +63,14 @@ _ActionT = TypeVar("_ActionT", bound=Action)
 _ArgumentParserT = TypeVar("_ArgumentParserT", bound=ArgumentParser)
 _N = TypeVar("_N")
 
-ONE_OR_MORE: str
-OPTIONAL: str
-PARSER: str
-REMAINDER: str
-SUPPRESS: str
-ZERO_OR_MORE: str
+ONE_OR_MORE: Literal["+"]
+OPTIONAL: Literal["?"]
+PARSER: Literal["A..."]
+REMAINDER: Literal["..."]
+_SUPPRESS_T = NewType("_SUPPRESS_T", str)
+SUPPRESS: _SUPPRESS_T | str  # not using Literal because argparse sometimes compares SUPPRESS with is
+# the | str is there so that foo = argparse.SUPPRESS; foo = "test" checks out in mypy
+ZERO_OR_MORE: Literal["*"]
 _UNRECOGNIZED_ARGS_ATTR: str  # undocumented
 
 class ArgumentError(Exception):
@@ -89,8 +106,11 @@ class _ActionsContainer:
     def add_argument(
         self,
         *name_or_flags: str,
-        action: str | type[Action] = ...,
-        nargs: int | str = ...,
+        action: Literal[
+            "store", "store_const", "store_true", "store_false", "append", "append_const", "count", "help", "version", "extend"
+        ]
+        | type[Action] = ...,
+        nargs: int | Literal["?", "*", "+", "...", "A...", "==SUPPRESS=="] | _SUPPRESS_T = ...,
         const: Any = ...,
         default: Any = ...,
         type: Callable[[str], _T] | FileType = ...,