Add py.typed marker

[sh-at-cs]

As discussed in #50, inline types have been added to the public interface by now.

However, they are still not usable by type checkers due to the absence of a py.typed marker. E.g. for a file like

from mjml import mjml_to_html

mjml_to_html()

Mypy complains:

mails.py:1: error: Skipping analyzing "mjml": module is installed, but missing library stubs or py.typed marker  [import-untyped]
mails.py:1: note: See https://mypy.readthedocs.io/en/stable/running_mypy.html#missing-imports
Found 1 error in 1 file (checked 1 source file)

This PR adds the py.typed marker file to let type checkers understand that they can actually use the type annotations provided by the package.