fix: require google-auth>=1.25.0 (#190)

diff --git a/setup.py b/setup.py
index 5de5aaf..26fab7e 100644
--- a/setup.py
+++ b/setup.py
@@ -31,7 +31,7 @@
 dependencies = [
     "googleapis-common-protos >= 1.6.0, < 2.0dev",
     "protobuf >= 3.12.0",
-    "google-auth >= 1.21.1, < 2.0dev",
+    "google-auth >= 1.25.0, < 2.0dev",
     "requests >= 2.18.0, < 3.0.0dev",
     "setuptools >= 40.3.0",
     "packaging >= 14.3",