I took a peek at the source for requests and found this comment:
This is supported by the docs as well: https://requests.readthedocs.io/en/stable/user/advanced/#ca-certificates which states that requests merely relies on the certifi package.
so, you can look at the certifi source and figure out how to monkey patch the where function.
https://github.com/certifi/python-certifi/blob/master/certifi/core.py
I don't see anything in the certifi source that reads from a place other than the pem file packaged with it.
Perhaps you and your client have different versions of python/certfi installed on your systems and aligning them would help?
Thanks, Mark