Specify packages using include rather than exclude

[cjwatson]

This avoids possible duplication if a wheel is built more than once from the same directory.

I ran into this when upgrading Debian's packaging of cssutils; we're currently in a transitional period where we're building for both Python 3.12 and 3.13, and this resulted in the second of the two wheels having a stray build directory. Using include rather than exclude avoids this, and seems to be simpler anyway.