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