include "examples" directory in the sdist again

[dne]

When moving to flit from setuptools, the "examples" directory was lost from the source distribution. I'm assuming that this was unintentional, since pyproject.toml on the main branch still includes it.

Fixes regression introduced in #1968.

[davidism]

Thanks, but I don't think we need to include them. The examples aren't part of the code docs or tests (and those last two don't technically need to be there either).

[dne]

Sure, they're not strictly necessary, but they've been part of the sdist since version 2.0 up until 3.1.3. And, as I mentioned, this regression seems specific for the 3.1.x branch.

FWIW, I discovered this when packaging 3.1.4 for Alpine Linux, which installs the examples as part of the "doc" sub-package (and so does e.g. Debian apparently)