Pin black version

[kylebarron]

CI on master is currently failing with

black....................................................................Failed
- hook id: black
- exit code: 123
- files were modified by this hook

reformatted /home/runner/work/rio-tiler/rio-tiler/rio_tiler/expression.py
reformatted /home/runner/work/rio-tiler/rio-tiler/docs/scripts/colormap_thumb.py
reformatted /home/runner/work/rio-tiler/rio-tiler/rio_tiler/io/base.py
error: cannot format /home/runner/work/rio-tiler/rio-tiler/rio_tiler/io/cogeo.py: INTERNAL ERROR: Black produced code that is not equivalent to the source.  Please report a bug on https://github.com/psf/black/issues.  This diff might be helpful: /tmp/blk_lzqera7f.log
error: cannot format /home/runner/work/rio-tiler/rio-tiler/rio_tiler/io/stac.py: INTERNAL ERROR: Black produced code that is not equivalent to the source.  Please report a bug on https://github.com/psf/black/issues.  This diff might be helpful: /tmp/blk_5b3wznne.log
reformatted /home/runner/work/rio-tiler/rio-tiler/rio_tiler/tasks.py
reformatted /home/runner/work/rio-tiler/rio-tiler/rio_tiler/reader.py
reformatted /home/runner/work/rio-tiler/rio-tiler/tests/test_io_cogeo.py
Oh no! 💥 💔 💥
6 files reformatted, 27 files left unchanged, 2 files failed to reformat.

This may be related to psf/black#1629.

In any case, good to pin the black version.