I had the same issue with biopython in finding the centre of mass. Although as Mensurj Dlakic suggested, there is a Struct class within Bio, this is an experimental class from JoaoRodrigues which does not appear to be implemented in any of the current versions of biopython: you can't simply "conda/pip install biopython" to access this module, but rather define it within your directory to use it.
The file which will enable you to call center_of_mass is here. Once you download this file in your directory, you can write a separate file which calls:
from Geometry import *
and proceed to call center_of_mass on any "structural object" such as the model/chain/residue of interest.
Although you can read through how Geometry does the center-of-mass calculation yourself, a summary is:
Find all the atoms of the structure, create a list of the atom masses, define the center of mass=(sum each set of xyz coordinate * respective atom mass)/(sum of each atomic mass), just as you would expect per a center of mass calculation in three dimensions.