chore: release 1.28.1 (#730)

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 603212e..1faab4d 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__ = "1.28.0"
+__version__ = "1.28.1"