Update lxml dependency (#427)
Update lxml dependency (#427)

* Update lxml dependency version in requirements.txt
* Update lxml dependency version in sdk/python/core/setup.py
* Update lxml dependency version in sdk/python/packages/setup.py

psykokwak4/ydk-gen,psykokwak4/ydk-gen,CiscoDevNet/ydk-gen,psykokwak4/ydk-gen,psykokwak4/ydk-gen,CiscoDevNet/ydk-gen,CiscoDevNet/ydk-gen,CiscoDevNet/ydk-gen,CiscoDevNet/ydk-gen,psykokwak4/ydk-gen,CiscoDevNet/ydk-gen