chore: declare that pandora is typed

Due to the change of layout, mypy considers pandora to be
an external library so I added the `py.typed` [marker](https://mypy.readthedocs.io/en/stable/running_mypy.html#missing-imports).
I had to change `meson.build` in order the file `py.typed` to be
included into the wheel so that when used as a library, pandora will be
declared typed.

Unfortunately, mypy and pylint still does not found pandora when
checking tests. This bug is referenced for [setuptools](python/mypy#13392), it could be the
same for meson-python but tagged as `wontfix`:

mesonbuild/meson-python#555

Author: PhML

meson.build

@@ -18,11 +18,11 @@ pybind11_dep = dependency('pybind11')
 doctest_dep = dependency('doctest', required: get_option('build_cpp_tests'))
 python_dep = py.dependency(embed: true, required: get_option('build_cpp_tests'))
 
-install_subdir(
-    'src/pandora',
-    install_dir: py.get_install_dir() / 'pandora',
-    strip_directory: true,
-)
+install_dir = py.get_install_dir() / 'pandora'
+
+install_subdir('src/pandora', install_dir: install_dir, strip_directory: true)
+
+install_data('src/pandora/py.typed', install_dir: install_dir)
 
 subdir('src/pandora/cpp')