[3.11] gh-112559: Avoid unnecessary conversion attempts to enum_klass in signal.py (GH-113040) (#113444)

gh-112559: Avoid unnecessary conversion attempts to enum_klass in signal.py (GH-113040)
(cherry picked from commit 050783cb37d6a09d8238fa640814df8a915f6a68)

Co-authored-by: Yilei Yang <yileiyang@google.com>