The full details are discussed in this github issue: https://github.com/Azure/azure-sdk-for-python/issues/3447.
PyPI uses `python_requires` metadata to communicate that a python package is only required for a particular version of python. It appears that nexus is not acting on this metadata so when I do a pip install packages are being pulled which shouldn't be which prevents the install from completing.
As detailed in the github issue there's a relatively simple workaround for my current problem. However I'm unsure these workarounds will work with more complex dependencies.