Add py.typed marker

[trevorbaca]

Abjad includes extensive typehints that can be read by typecheckers like mypy.

Users of Abjad 3.7 (and earlier) have to tell mypy where Abjad is installed by setting the MYPYPATH environment variable. But this is silly because mypy already knows the location installed packages, including Abjad.

Including an (empty) py.typed file in Abjad's source directory fixes this. And Abjad 3.8 users will no longer need to set MYPYPATH to use mypy with Abjad.