gh-129205: Modernize test_eintr (#129316)

* Use f-string.
* Fix grammar: replace 'datas' with 'data' (and replace 'data' with
  'item').
* Remove unused variables: 'pid' and 'old_mask'.
* Factorize test_read() and test_readinto() code.

Co-authored-by: Cody Maloney <cmaloney@users.noreply.github.com>