typing: Add py.typed file and minimal *.pyi example

MANIFEST.in

@@ -1,6 +1,8 @@
 include README.rst
 include LICENSE
+include skyfield/py.typed
 include skyfield/data/*.gz
 include skyfield/data/*.npy
 include skyfield/data/*.npz
 include skyfield/tests/data/*
+global-include *.pyi

setup.py

@@ -33,6 +33,7 @@
         'skyfield.tests',
     ],
     package_data = {
+        'skyfield': ['py.typed', '*.pyi'],
         'skyfield.data': ['*.gz', '*.npy', '*.npz'],
         'skyfield.tests': ['data/*'],
     },

skyfield/naifcodes.pyi

@@ -0,0 +1,6 @@
+code_name_pairs: list[tuple[int, str]]
+code_names: dict[int, str]
+name_codes: dict[str, int]
+
+def numbered_name_of(code: int) -> str: ...
+def _target_name(target: str | int) -> str: ...