gh-116738: Make _codecs module thread-safe (#117530)

The module itself is a thin wrapper around calls to functions in
`Python/codecs.c`, so that's where the meaningful changes happened:

- Move codecs-related state that lives on `PyInterpreterState` to a
  struct declared in `pycore_codecs.h`.

- In free-threaded builds, add a mutex to `codecs_state` to synchronize
  operations on `search_path`. Because `search_path_mutex` is used as a
  normal mutex and not a critical section, we must be extremely careful
  with operations called while holding it.

- The codec registry is explicitly initialized as part of
  `_PyUnicode_InitEncodings` to simplify thread-safety.

Include/internal/pycore_codecs.h

@@ -8,6 +8,17 @@ extern "C" {
 #  error "this header requires Py_BUILD_CORE define"
 #endif
 
+#include "pycore_lock.h"          // PyMutex
+
+/* Initialize codecs-related state for the given interpreter, including
+   registering the first codec search function. Must be called before any other
+   PyCodec-related functions, and while only one thread is active. */
+extern PyStatus _PyCodec_InitRegistry(PyInterpreterState *interp);
+
+/* Finalize codecs-related state for the given interpreter. No PyCodec-related
+   functions other than PyCodec_Unregister() may be called after this. */
+extern void _PyCodec_Fini(PyInterpreterState *interp);
+
 extern PyObject* _PyCodec_Lookup(const char *encoding);
 
 /* Text codec specific encoding and decoding API.
@@ -48,6 +59,26 @@ extern PyObject* _PyCodecInfo_GetIncrementalEncoder(
    PyObject *codec_info,
    const char *errors);
 
+// Per-interpreter state used by codecs.c.
+struct codecs_state {
+    // A list of callable objects used to search for codecs.
+    PyObject *search_path;
+
+    // A dict mapping codec names to codecs returned from a callable in
+    // search_path.
+    PyObject *search_cache;
+
+    // A dict mapping error handling strategies to functions to implement them.
+    PyObject *error_registry;
+
+#ifdef Py_GIL_DISABLED
+    // Used to safely delete a specific item from search_path.
+    PyMutex search_path_mutex;
+#endif
+
+    // Whether or not the rest of the state is initialized.
+    int initialized;
+};
 
 #ifdef __cplusplus
 }