Bump downstream dependencies in CI (#14012)

Co-authored-by: pyca-boringbot[bot] <pyca-boringbot[bot][email protected]>
diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index fbb87b2..c58dea0 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -468,8 +468,8 @@
             PATH: scapy
           - DOWNSTREAM: sigstore-python
             REPO: sigstore/sigstore-python
-            # Latest commit on the sigstore-python main branch, as of Dec 06, 2025.
-            REF: 4334ac8d481e09977b9932d3b3f58190345b6af2
+            # Latest commit on the sigstore-python main branch, as of Dec 13, 2025.
+            REF: 6b27f06377df5349bc666247dedaa1b7c0de05e2
             PATH: sigstore-python
     name: "Downstream tests for ${{ matrix.DOWNSTREAM }}"
     timeout-minutes: 15