ext: Upgrade PyBind11 to version 2.1.1Change-Id: I16870dec402d661295f9d013dc23e362b2b2c169Signed-off-by: Andreas Sandberg <andreas.sandberg@arm.com>Reviewed-by: Curtis Dunham <curtis.dunham@arm.com>Reviewed-on: https://gem5-review.googlesource.com/3225Reviewed-by: Jason Lowe-Power <jason@lowepower.com>