add __type_checking__ keyword

Author: methane

Doc/reference/lexical_analysis.rst

@@ -348,6 +348,7 @@ exactly as written here:
    as         def        from       nonlocal   while
    assert     del        global     not        with
    async      elif       if         or         yield
+   __type_checking__
 
 
 .. _soft-keywords:

Grammar/python.gram

@@ -848,6 +848,7 @@ atom[expr_ty]:
     | NAME
     | 'True' { _PyAST_Constant(Py_True, NULL, EXTRA) }
     | 'False' { _PyAST_Constant(Py_False, NULL, EXTRA) }
+    | '__type_checking__' { _PyAST_Constant(Py_False, NULL, EXTRA) }
     | 'None' { _PyAST_Constant(Py_None, NULL, EXTRA) }
     | &(STRING|FSTRING_START) strings
     | NUMBER
@@ -1154,7 +1155,7 @@ invalid_arguments:
         RAISE_SYNTAX_ERROR_KNOWN_RANGE(a, _PyPegen_get_last_comprehension_item(PyPegen_last_item(b, comprehension_ty)), "Generator expression must be parenthesized") }
     | a=args ',' args { _PyPegen_arguments_parsing_error(p, a) }
 invalid_kwarg:
-    | a[Token*]=('True'|'False'|'None') b='=' {
+    | a[Token*]=('True'|'False'|'None'|'__type_checking__') b='=' {
         RAISE_SYNTAX_ERROR_KNOWN_RANGE(a, b, "cannot assign to %s", PyBytes_AS_STRING(a->bytes)) }
     | a=NAME b='=' expression for_if_clauses {
         RAISE_SYNTAX_ERROR_KNOWN_RANGE(a, b, "invalid syntax. Maybe you meant '==' or ':=' instead of '='?")}
@@ -1209,7 +1210,7 @@ invalid_named_expression(memo):
             a, "cannot use assignment expressions with %s", _PyPegen_get_expr_name(a)) }
     | a=NAME '=' b=bitwise_or !('='|':=') {
         RAISE_SYNTAX_ERROR_KNOWN_RANGE(a, b, "invalid syntax. Maybe you meant '==' or ':=' instead of '='?") }
-    | !(list|tuple|genexp|'True'|'None'|'False') a=bitwise_or b='=' bitwise_or !('='|':=') {
+    | !(list|tuple|genexp|'True'|'None'|'False'|'__type_checking__') a=bitwise_or b='=' bitwise_or !('='|':=') {
         RAISE_SYNTAX_ERROR_KNOWN_LOCATION(a, "cannot assign to %s here. Maybe you meant '==' instead of '='?",
                                           _PyPegen_get_expr_name(a)) }