bpo-40939: Remove the old parser (GH-20768)

This commit removes the old parser, the deprecated parser module, the old parser compatibility flags and environment variables and all associated support code and documentation.

Parser/string_parser.h

@@ -0,0 +1,46 @@
+#ifndef STRINGS_H
+#define STRINGS_H
+
+#include <Python.h>
+#include <Python-ast.h>
+#include "pegen.h"
+
+#define EXPRLIST_N_CACHED  64
+
+typedef struct {
+    /* Incrementally build an array of expr_ty, so be used in an
+       asdl_seq. Cache some small but reasonably sized number of
+       expr_ty's, and then after that start dynamically allocating,
+       doubling the number allocated each time. Note that the f-string
+       f'{0}a{1}' contains 3 expr_ty's: 2 FormattedValue's, and one
+       Constant for the literal 'a'. So you add expr_ty's about twice as
+       fast as you add expressions in an f-string. */
+
+    Py_ssize_t allocated;  /* Number we've allocated. */
+    Py_ssize_t size;       /* Number we've used. */
+    expr_ty    *p;         /* Pointer to the memory we're actually
+                              using. Will point to 'data' until we
+                              start dynamically allocating. */
+    expr_ty    data[EXPRLIST_N_CACHED];
+} ExprList;
+
+/* The FstringParser is designed to add a mix of strings and
+   f-strings, and concat them together as needed. Ultimately, it
+   generates an expr_ty. */
+typedef struct {
+    PyObject *last_str;
+    ExprList expr_list;
+    int fmode;
+} FstringParser;
+
+void _PyPegen_FstringParser_Init(FstringParser *);
+int _PyPegen_parsestr(Parser *, int *, int *, PyObject **,
+                      const char **, Py_ssize_t *, Token *);
+int _PyPegen_FstringParser_ConcatFstring(Parser *, FstringParser *, const char **,
+                                const char *, int, int, Token *, Token *,
+                                Token *);
+int _PyPegen_FstringParser_ConcatAndDel(FstringParser *, PyObject *);
+expr_ty _PyPegen_FstringParser_Finish(Parser *, FstringParser *, Token *, Token *);
+void _PyPegen_FstringParser_Dealloc(FstringParser *);
+
+#endif