Add black as a dependency for Python code

This patch adds the current `HEAD` of `main` for the Python formatter
`black` to our dependencies.  `black` used to be a transitive
dependency, but this was removed as part of
b390620.  The decision to use `HEAD`
instead of a particular version is only to match the version used in CI
in `.github/formatting-check.yaml`; if we elect to use a particular
version, we should do so there as well.

Signed-off-by: Patrick M. Niedzielski <[email protected]>

src/python/requirements.txt

@@ -1,3 +1,4 @@
+black @ git+https://github.com/psf/black
 boofuzz
 crc32c
 docker