add toml to manifest

[bjlittle]

This pull-request adds *.toml files to the sdist manifest.

[HGWright]

nice spot to fix the failures.

[trexfeathers]

@bjlittle perhaps you can educate me: I would have expected to exclude .toml files, since they won't be required as part of a user install?

I note that Iris excludes .ruff.toml specifically, and makes no mention at all of pyproject.toml, suggesting that maybe some special rules apply there.

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Note that following two similar PR's, we now have this:

iris-grib/MANIFEST.in

Lines 13 to 16 in 71703ae

	exclude .ruff.toml
	include *.md
	include *.py
	include *.toml

I knew I should have checked main before merging #435!