Use Prettier pre-commit mirror (#1896)
Use Prettier pre-commit mirror (#1896)

This fixes Prettier install failures similar to those seen in
https://github.com/prettier/prettier/issues/9459, and is the solution
recommended there.

Signed-off-by: Paul "TBBle" Hampson <220d126b3d70d9d7b0fda508e4cb236265bccc41@Pobox.com>
psf/black