internals: optimize std::unordered_map internals with noexcept (#5960)

include/pybind11/detail/internals.h

diff --git a/include/pybind11/detail/internals.h b/include/pybind11/detail/internals.h
index 41585cf..d719597 100644
--- a/include/pybind11/detail/internals.h
+++ b/include/pybind11/detail/internals.h
@@ -191,7 +191,7 @@
 }
 
 struct type_hash {
-    size_t operator()(const std::type_index &t) const {
+    size_t operator()(const std::type_index &t) const noexcept {
         size_t hash = 5381;
         const char *ptr = t.name();
         while (auto c = static_cast<unsigned char>(*ptr++)) {
@@ -202,7 +202,7 @@
 };
 
 struct type_equal_to {
-    bool operator()(const std::type_index &lhs, const std::type_index &rhs) const {
+    bool operator()(const std::type_index &lhs, const std::type_index &rhs) const noexcept {
         return lhs.name() == rhs.name() || std::strcmp(lhs.name(), rhs.name()) == 0;
     }
 };
@@ -218,7 +218,7 @@
 using type_map = std::unordered_map<std::type_index, value_type, type_hash, type_equal_to>;
 
 struct override_hash {
-    size_t operator()(const std::pair<const PyObject *, const char *> &v) const {
+    size_t operator()(const std::pair<const PyObject *, const char *> &v) const noexcept {
         size_t value = std::hash<const void *>()(v.first);
         value ^= std::hash<const void *>()(v.second) + 0x9e3779b9 + (value << 6) + (value >> 2);
         return value;