test: fix the usage so the tests can pass in g3 (#498)
* fix: fix the usage so the tests can pass in g3
* use frozenset
diff --git a/google/auth/jwt.py b/google/auth/jwt.py
index 9248eb2..24b92eb 100644
--- a/google/auth/jwt.py
+++ b/google/auth/jwt.py
@@ -67,7 +67,7 @@
_DEFAULT_TOKEN_LIFETIME_SECS = 3600 # 1 hour in seconds
_DEFAULT_MAX_CACHE_SIZE = 10
_ALGORITHM_TO_VERIFIER_CLASS = {"RS256": crypt.RSAVerifier}
-_CRYPTOGRAPHY_BASED_ALGORITHMS = set(["ES256"])
+_CRYPTOGRAPHY_BASED_ALGORITHMS = frozenset(["ES256"])
if es256 is not None: # pragma: NO COVER
_ALGORITHM_TO_VERIFIER_CLASS["ES256"] = es256.ES256Verifier