[gdb/python] Introduce py_{none,true,false,notimplemented} functions

I came across this code:
...
   if (c)
     return v;
   else
     Py_RETURN_NONE;
...
and realized I couldn't easily rewrite it into "return c ? v : ...".

Probably something like this would work:
...
  return c ? v : ([] { Py_RETURN_NONE; } ());
...
but it made me wonder if we can get rid of Py_RETURN_NONE.

The only point of it seems to be to increase the reference count on the
Py_None object for older python versions.

Add a refcount-safe wrapper py_none (returning a gdbpy_ref), with the aim of
replacing all uses of Py_RETURN_NONE with "return py_none ().release ()".

Likewise for Py_RETURN_TRUE/Py_RETURN_FALSE/Py_RETURN_NOTIMPLEMENTED.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=34145

gdb/python/python-internal.h

diff --git a/gdb/python/python-internal.h b/gdb/python/python-internal.h
index 3147156..5548c45 100644
--- a/gdb/python/python-internal.h
+++ b/gdb/python/python-internal.h
@@ -1343,4 +1343,35 @@ class gdbpy_memoizing_registry_storage
 extern int eval_python_command (const char *command, int start_symbol,
 				const char *filename = nullptr);
 
+/* The following four functions are refcount-safe wrappers around
+   Py_RETURN_{NONE,TRUE,FALSE,NOTIMPLEMENTED}.  */
+
+static inline gdbpy_ref<>
+py_none ()
+{
+  auto f = [] { Py_RETURN_NONE; };
+  return gdbpy_ref<> (f ());
+}
+
+static inline gdbpy_ref<>
+py_true ()
+{
+  auto f = [] { Py_RETURN_TRUE; };
+  return gdbpy_ref<> (f ());
+}
+
+static inline gdbpy_ref<>
+py_false ()
+{
+  auto f = [] { Py_RETURN_FALSE; };
+  return gdbpy_ref<> (f ());
+}
+
+static inline gdbpy_ref<>
+py_notimplemented ()
+{
+  auto f = [] { Py_RETURN_NOTIMPLEMENTED; };
+  return gdbpy_ref<> (f ());
+}
+
 #endif /* GDB_PYTHON_PYTHON_INTERNAL_H */