chore: release 1.31.0 (#771)

Co-authored-by: release-please[bot] <55107282+release-please[bot]@users.noreply.github.com>
diff --git a/google/auth/version.py b/google/auth/version.py
index 5e7a79c..a8d5599 100644
--- a/google/auth/version.py
+++ b/google/auth/version.py
@@ -12,4 +12,4 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-__version__ = "1.30.2"
+__version__ = "1.31.0"