Resolve new Clang dev C++11 errors:

```
The CXX compiler identification is Clang 17.0.0
```

```
pytypes.h:1615:23: error: identifier '_s' preceded by whitespace in a literal operator declaration is deprecated [-Werror,-Wdeprecated-literal-operator]
```

```
cast.h:1380:26: error: identifier '_a' preceded by whitespace in a literal operator declaration is deprecated [-Werror,-Wdeprecated-literal-operator]
```

Author: rwgk

include/pybind11/cast.h

@@ -1377,7 +1377,7 @@ inline namespace literals {
 /** \rst
     String literal version of `arg`
  \endrst */
-constexpr arg operator"" _a(const char *name, size_t) { return arg(name); }
+constexpr arg operator""_a(const char *name, size_t) { return arg(name); }
 } // namespace literals
 
 PYBIND11_NAMESPACE_BEGIN(detail)

include/pybind11/pytypes.h

@@ -1612,7 +1612,7 @@ inline namespace literals {
 /** \rst
     String literal version of `str`
  \endrst */
-inline str operator"" _s(const char *s, size_t size) { return {s, size}; }
+inline str operator""_s(const char *s, size_t size) { return {s, size}; }
 } // namespace literals
 
 /// \addtogroup pytypes