[3.13] gh-119729: Use 't' in pkg-config file name for free-threaded build (GH-119738) (#119797)

For example, the free-threaded build now generates
`lib/pkgconfig/python-3.13t.pc` and the debug build generates
`lib/pkgconfig/python-3.13d.pc`.
(cherry picked from commit 1c04c63ced5038e8f45a2aac7dc45f0815a4ddc5)

Co-authored-by: Sam Gross <colesbury@gmail.com>