Starting from py3.14 pkgutil.get_loader was removed(*) and so pygolang
can not even start building:

    (py314.venv) pygolang$ python setup.py build_ext -i
    Traceback (most recent call last):
      File "/home/kirr/src/tools/go/pygolang-master/setup.py", line 226, in <module>
        x_dsos      = [DSO('golang.runtime.libgolang',
                       ~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^
                            ['golang/runtime/libgolang.cpp',
                            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
        ...<33 lines>...
                            define_macros   = [('BUILDING_LIBGOLANG', None)],
                            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
                            soversion       = '0.1'),
                            ^^^^^^^^^^^^^^^^^^^^^^^^
      File "/home/kirr/src/tools/go/pygolang-master/golang/pyx/build.py", line 235, in DSO
        _, kw = _with_build_defaults(name, kw)
                ~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^
      File "/home/kirr/src/tools/go/pygolang-master/golang/pyx/build.py", line 244, in _with_build_defaults
        gopkg = _findpkg("golang")
      File "/home/kirr/src/tools/go/pygolang-master/golang/pyx/build.py", line 134, in _findpkg
        pkg = pkgutil.get_loader(pkgname)
              ^^^^^^^^^^^^^^^^^^
    AttributeError: module 'pkgutil' has no attribute 'get_loader'

-> Fix it by replacing pkgutil.get_loader with importlib.util.find_spec .

Need to adjust trun to inject golang.__spec__ in addition to
golang.__loader__ because else find_spec cannot find the golang module.

(*) https://docs.python.org/3/whatsnew/3.14.html#pkgutil

/reviewed-by @jerome
/reviewed-on !39