BLD: clean-up sdist creation
I messed up in https://github.com/pandas-dev/pandas/pull/28374/files#diff-2eeaed663bd0d25b7e608891384b7298R531. I meant to revert the lines that excluded cythonized files in the sdist to save it for a followup.

So I'm using this to clean up the sdist logic a bit, and exclude a few test data files that have snuck into our distribution.
LGTM @jorisvandenbossche done and passing. Whoops, this should have a release note. 1 sec.