PythonAPI: Fix segfault in GetAvailableMaps

When using CARLA with Python 3.10, I'm getting a segfault in
GetAvailableMaps. The problem disappears when PyList manipulation does
not happen with GIL unlocked, as done in this commit.

The initial part of crash backtrace (from GDB) is below:

    Program terminated with signal SIGSEGV, Segmentation fault.

    warning: Section `.reg-xstate/49253' in core file too small.
    #0  _PyInterpreterState_GET () at ./Include/internal/pycore_pystate.h:117
    117	   return tstate->interp;
    [Current thread is 1 (Thread 0x7fe6fe48f740 (LWP 49253))]
    (gdb) bt
    #0  _PyInterpreterState_GET () at ./Include/internal/pycore_pystate.h:117
    matejm42#1  get_list_state () at Objects/listobject.c:26
    carla-simulator#2  PyList_New (size=0) at Objects/listobject.c:159
    carla-simulator#3  0x00007fe6fdc0dab0 in boost::python::detail::list_base::list_base() () from /nix/store/c95f3nrkz3sflvycihyw1c8q4nk47p4m-boost-1.79.0/lib/libboost_python310.so.1.79.0
    carla-simulator#4  0x00007fe6ef9ecfc4 in boost::python::list::list (this=0x7ffd8a8aae28) at include/boost/python/list.hpp:61
    carla-simulator#5  GetAvailableMaps (self=...) at source/libcarla/Client.cpp:26
    carla-simulator#6  0x00007fe6efb6a8fe in boost::python::detail::invoke<boost::python::to_python_value<boost::python::list const&>, boost::python::list (*)(carla::client::Client const&), boost::python::arg_from_python<carla::client::Client const&> > (rc=..., f=<optimized out>, ac0=...)
        at include/boost/python/detail/invoke.hpp:73
    carla-simulator#7  boost::python::detail::caller_arity<1u>::impl<boost::python::list (*)(carla::client::Client const&), boost::python::default_call_policies, boost::mpl::vector2<boost::python::list, carla::client::Client const&> >::operator() (args_=<optimized out>, this=<optimized out>)
        at include/boost/python/detail/caller.hpp:233
    carla-simulator#8  boost::python::objects::caller_py_function_impl<boost::python::detail::caller<boost::python::list (*)(carla::client::Client const&), boost::python::default_call_policies, boost::mpl::vector2<boost::python::list, carla::client::Client const&> > >::operator() (
        this=<optimized out>, args=<optimized out>, kw=<optimized out>) at include/boost/python/object/py_function.hpp:38
    carla-simulator#9  0x00007fe6fdc1b4dd in boost::python::objects::function::call(_object*, _object*) const () from /nix/store/c95f3nrkz3sflvycihyw1c8q4nk47p4m-boost-1.79.0/lib/libboost_python310.so.1.79.0
    carla-simulator#10 0x00007fe6fdc1b6a8 in boost::detail::function::void_function_ref_invoker0<boost::python::objects::(anonymous namespace)::bind_return, void>::invoke(boost::detail::function::function_buffer&) ()
       from /nix/store/c95f3nrkz3sflvycihyw1c8q4nk47p4m-boost-1.79.0/lib/libboost_python310.so.1.79.0
    ...

PythonAPI/carla/source/libcarla/Client.cpp

@@ -22,9 +22,13 @@ static void SetTimeout(carla::client::Client &client, double seconds) {
 }
 
 static auto GetAvailableMaps(const carla::client::Client &self) {
-  carla::PythonUtil::ReleaseGIL unlock;
   boost::python::list result;
-  for (const auto &str : self.GetAvailableMaps()) {
+  std::vector<std::string> maps;
+  {
+    carla::PythonUtil::ReleaseGIL unlock;
+    maps = self.GetAvailableMaps();
+  }
+  for (const auto &str : maps) {
     result.append(str);
   }
   return result;