Upgrade pybind11 to 'v2.6.1'

"Combining older versions of pybind11 (< 2.6.0) with Python 3.9.0 will
trigger undefined behavior that typically manifests as crashes during
interpreter shutdown (but could also destroy your data. You have been
warned.)"

Change-Id: Ide85f8e710f46a3deaa500fff2ad496ea4bb25fd
diff --git a/METADATA b/METADATA
index bbcbe08..05c3ffb 100644
--- a/METADATA
+++ b/METADATA
@@ -9,11 +9,11 @@
     type: GIT
     value: "https://github.com/pybind/pybind11.git"
   }
-  version: "v2.5.0"
+  version: "v2.6.1"
   license_type: NOTICE
   last_upgrade_date {
-    year: 2020
-    month: 3
-    day: 31
+    year: 2021
+    month: 1
+    day: 13
   }
 }