[3.11] gh-109237: Fix test_site for non-ASCII working directory (GH-109238) (#109240)

gh-109237: Fix test_site for non-ASCII working directory (GH-109238)

Fix test_site.test_underpth_basic() when the working directory
contains at least one non-ASCII character: encode the "._pth" file to
UTF-8 and enable the UTF-8 Mode to use UTF-8 for the child process
stdout.
(cherry picked from commit cbb3a6f8ada3d133c3ab9f9465b65067fce5bb42)

Co-authored-by: Victor Stinner <vstinner@python.org>