[3.13] gh-125268: Use static string for "1e309" in AST (GH-125272) (GH-125280)

When formatting the AST as a string, infinite values are replaced by
1e309, which evaluates to infinity. The initialization of this string
replacement was not thread-safe in the free threading build.
(cherry picked from commit 427dcf24de4e06d239745d74d08c4b2e541dca5a)