bpo-47152: Automatically regenerate sre_constants.h (GH-91439)

* Move the code for generating Modules/_sre/sre_constants.h from
  Lib/re/_constants.py into a separate script
  Tools/scripts/generate_sre_constants.py.
* Add target `regen-sre` in the makefile.
* Make target `regen-all` depending on `regen-sre`.

Tools/scripts/generate_sre_constants.py

@@ -0,0 +1,62 @@
+#! /usr/bin/env python3
+# This script generates Modules/_sre/sre_constants.h from Lib/re/_constants.py.
+
+
+def update_file(file, content):
+    try:
+        with open(file, 'r') as fobj:
+            if fobj.read() == content:
+                return False
+    except (OSError, ValueError):
+        pass
+    with open(file, 'w') as fobj:
+        fobj.write(content)
+    return True
+
+sre_constants_header = """\
+/*
+ * Secret Labs' Regular Expression Engine
+ *
+ * regular expression matching engine
+ *
+ * Auto-generated by Tools/scripts/generate_sre_constants.py from
+ * Lib/re/_constants.py.
+ *
+ * Copyright (c) 1997-2001 by Secret Labs AB.  All rights reserved.
+ *
+ * See the sre.c file for information on usage and redistribution.
+ */
+
+"""
+
+def main(infile='Lib/re/_constants.py', outfile='Modules/_sre/sre_constants.h'):
+    ns = {}
+    with open(infile) as fp:
+        code = fp.read()
+    exec(code, ns)
+
+    def dump(d, prefix):
+        items = sorted(d)
+        for item in items:
+            yield "#define %s_%s %d\n" % (prefix, item, item)
+
+    def dump2(d, prefix):
+        items = [(value, name) for name, value in d.items()
+                 if name.startswith(prefix)]
+        for value, name in sorted(items):
+            yield "#define %s %d\n" % (name, value)
+
+    content = [sre_constants_header]
+    content.append("#define SRE_MAGIC %d\n" % ns["MAGIC"])
+    content.extend(dump(ns["OPCODES"], "SRE_OP"))
+    content.extend(dump(ns["ATCODES"], "SRE"))
+    content.extend(dump(ns["CHCODES"], "SRE"))
+    content.extend(dump2(ns, "SRE_FLAG_"))
+    content.extend(dump2(ns, "SRE_INFO_"))
+
+    update_file(outfile, ''.join(content))
+
+
+if __name__ == '__main__':
+    import sys
+    main(*sys.argv[1:])