devel/py-black: Really fix packaging issues

• When the port was updated via dea032cba8b3, it still wouldn't build, which was then fixed with 25b2be6023fe.
  
  Although the port can now be built, the content of the generated package is not correct at the moment as every Python module is placed into an additional "src/" directory.
  
  There was an attempt to fix the problem via 76670f6a4f84, but the root cause has not yet been eliminated because the Python module is still not usable:
  
  $ python3.9 -m black /usr/local/bin/python3.9: No module named black

• Remedy these issues by adjusting the backported "setup.py" to generate the correct package content.

• Bump PORTREVISION due package change.

PR: 267040 [1]
Reported by: Martin Birgmeier [1]

		pkg-fallout (for games/anki which requires py-black)

Approved by: portmgr (blanket, fix packaging issues)
Fixes: 25b2be6023fe devel/py-black: fix build