gh-140544: Always assume that thread locals are available (GH-140690)

Python has required thread local support since 3.12 (see GH-103324). By assuming that thread locals are always supported, we can improve the performance of third-party extensions by allowing them to access the attached thread and interpreter states directly.

Python/import.c

@@ -782,18 +782,13 @@ _PyImport_ClearModulesByIndex(PyInterpreterState *interp)
    substitute this (if the name actually matches).
 */
 
-#ifdef HAVE_THREAD_LOCAL
 _Py_thread_local const char *pkgcontext = NULL;
 # undef PKGCONTEXT
 # define PKGCONTEXT pkgcontext
-#endif
 
 const char *
 _PyImport_ResolveNameWithPackageContext(const char *name)
 {
-#ifndef HAVE_THREAD_LOCAL
-    PyMutex_Lock(&EXTENSIONS.mutex);
-#endif
     if (PKGCONTEXT != NULL) {
         const char *p = strrchr(PKGCONTEXT, '.');
         if (p != NULL && strcmp(name, p+1) == 0) {
@@ -801,23 +796,14 @@ _PyImport_ResolveNameWithPackageContext(const char *name)
             PKGCONTEXT = NULL;
         }
     }
-#ifndef HAVE_THREAD_LOCAL
-    PyMutex_Unlock(&EXTENSIONS.mutex);
-#endif
     return name;
 }
 
 const char *
 _PyImport_SwapPackageContext(const char *newcontext)
 {
-#ifndef HAVE_THREAD_LOCAL
-    PyMutex_Lock(&EXTENSIONS.mutex);
-#endif
     const char *oldcontext = PKGCONTEXT;
     PKGCONTEXT = newcontext;
-#ifndef HAVE_THREAD_LOCAL
-    PyMutex_Unlock(&EXTENSIONS.mutex);
-#endif
     return oldcontext;
 }
 

Python/pystate.c

@@ -67,9 +67,6 @@ to avoid the expense of doing their own locking).
    For each of these functions, the GIL must be held by the current thread.
  */
 
-#ifndef HAVE_THREAD_LOCAL
-#  error "no supported thread-local variable storage classifier"
-#endif
 
 /* The attached thread state for the current thread. */
 _Py_thread_local PyThreadState *_Py_tss_tstate = NULL;