Missing py.typed marker for mypy support

[rpanman-sonatype]

Pylightxl Version: 1.61 Python Version: 3.9

Summary of Bug/Feature: I use mypy as a static type checker and at the moment it's raising this problem with pylightxl:

$ mypy main.py 
/Users/rpanman/Coding/git/cs_ci_nps/main.py:6: error: Skipping analyzing "pylightxl": module is installed, but missing library stubs or py.typed marker  [import]
/Users/rpanman/Coding/git/cs_ci_nps/main.py:6: note: See https://mypy.readthedocs.io/en/stable/running_mypy.html#missing-imports
Found 1 error in 1 file (checked 1 source file)

I can see that #69 introduced type hinting so it looks like it's just the py.typed file that needs adding in.

Traceback:

None.

Suggestion for fix:

Add the py.typed file to the module as described in the mypy PEP-561 compatible packages instruction

I'm happy to raise a PR if that's easiest for you, just let me know.