Explicitly export exception types. (#2999)

* Set visibility of exceptions to default.

Co-authored-by: XZiar <czktc2007@gmail.com>

* add test

* update docs

* Skip failed test.

tests/test_exceptions.cpp

@@ -7,6 +7,7 @@
     BSD-style license that can be found in the LICENSE file.
 */
 
+#include "test_exceptions.h"
 #include "pybind11_tests.h"
 
 // A type that should be raised as an exception in Python
@@ -228,4 +229,5 @@ TEST_SUBMODULE(exceptions, m) {
     // Test repr that cannot be displayed
     m.def("simple_bool_passthrough", [](bool x) {return x;});
 
+    m.def("throw_should_be_translated_to_key_error", []() { throw shared_exception(); });
 }