IWYU cleanup

```
iwyu version: include-what-you-use 0.17 based on Debian clang version 13.0.1-3+build2
```

Command used:

```
iwyu -c -std=c++17 -DPYBIND11_TEST_BOOST -Iinclude/pybind11 -I/usr/include/python3.9 -I/usr/include/eigen3 include/pybind11/pytypes.cpp
```

pytypes.cpp is a temporary file: `#include "pytypes.h"`

The raw output is very long and noisy.

I decided to use `#include <cstddef>` instead of `#include <cstdio>` for `std::size_t` (iwyu sticks to the manual choice).

I ignored all iwyu suggestions that are indirectly covered by `#include <Python.h>`.

I manually verified that all added includes are actually needed.

include/pybind11/pytypes.h

@@ -12,10 +12,15 @@
 #include "detail/common.h"
 #include "buffer_info.h"
 
-#include <cstdio>
+#include <assert.h>
+#include <cstddef>
 #include <exception>
+#include <frameobject.h>
+#include <iterator>
+#include <memory>
 #include <string>
 #include <type_traits>
+#include <typeinfo>
 #include <utility>
 
 #if defined(PYBIND11_HAS_OPTIONAL)