[3.13] gh-117657: Fix TSAN reported race in `_PyEval_IsGILEnabled`. (GH-119921) (#119939)

The GIL may be disabled concurrently with this call so we need to use a
relaxed atomic load.
(cherry picked from commit f3b89a63cbb6d46e5ed40d5cd9813cdf9189ce35)

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