PyVmomi is available to download either though Python pip or as a system package from different repos.
For Python installation, use the following command:
pip install -U pyvmomi
Notice the version downloaded from pip is 6.5.2017.5-1, which correlates with the vSphere release VMware vSphere 6.5, but this doesn't mean it won't work with older releases of ESXi. For example, I have VMware vSphere 5.5, which works flawlessly with the latest pyvmomi version.
For system installation:
yum install pyvmomi -y