Log Message:
Update fonts/py-fonttools to version 3.15.1

Changelog for this release:
- [otConverters] Implemented ``__add__`` and ``__radd__`` methods on
  ``otConverters._LazyList`` that decompile a lazy list before adding
  it to another list or ``_LazyList`` instance. Fixes an ``AttributeError``
  in the ``subset`` module when attempting to sum ``_LazyList`` objects
  (6ef48bd2, 1aef1683).
- [AAT] Support the `opbd` table with optical bounds (a47f6588).
- [AAT] Support `prop` table with glyph properties (d05617b4).