make_tempfile() supports a content= parameter to specify content

going into the temp file without having to do a follow-up write.

However, TestCase.precreate_tempfiles() completely ignored this,

pre-creating the files but never writing the content to them when

handling the spy for make_tempfile().
This has been updated to perform the write when called, if content is

provided.
It also updates make_tempfile() for type safety and to require keyword

arguments (deprecating positional arguments), to help ensure signatures

stay consistent.

Unit tests pass on Python 3.7-3.11.