This is a topic where there are two points of view. While neither
is fundamentally better than the other in reality, what's best is
to not have any arguments about it. The tox.ini comments about 503
and 504 that were in place make the argument that:
- 503 is intended to be disabled and 504 enabled by default
- Donald Knuth believes 504 is the right way
Since Donald Knuth is smarter than all of us, align with 504, match
the comments in the file and turn on enforcement to keep it that way.
Change-Id: I92d4d1e82935e30ae42a0e14e641cbe36fd6e811