Log Message:
py-cffi: updated to 1.15.1

v1.15.1
=======

* If you call `ffi.embedding_api()` but don't write any `extern "Python"`
  function there, then the resulting C code would fail an assert.  Fixed.

* Updated Windows/arm64 embedded libffi static lib to v3.4.2, and scripted
  to ease future updates (thanks Niyas Sait!)