python pants ModuleNotFoundError when using python 3

Question

I am new to python and attempting to setup up a mono repo using pants as the build system. All has been going well, until I told pants to use a python 3 interpreter by setting in pants.ini

[python-setup]
interpreter_constraints: ["CPython>=3.6.5"]

I have a setup a local library python_library which is imported into a python_binary.

In other words, for an identical code base,

• running ./pants run ... on python 2.7 works perfectly
• running ./pants run ... with the above interpreter_constraints fails with ModuleNotFoundError

The stack trace I am getting looks like

Traceback (most recent call last):
  File "/home/daniel/PycharmProjects/analytics-platform/.pants.d/run/py/CPython-3.6.8/8c5d2b01b2fb9952ae4d6116e537a04edd7039e8/.bootstrap/pex/pex.py", line 397, in execute
    exit_code = self._wrap_coverage(self._wrap_profiling, self._execute)
  File "/home/daniel/PycharmProjects/analytics-platform/.pants.d/run/py/CPython-3.6.8/8c5d2b01b2fb9952ae4d6116e537a04edd7039e8/.bootstrap/pex/pex.py", line 329, in _wrap_coverage
    return runner(*args)
  File "/home/daniel/PycharmProjects/analytics-platform/.pants.d/run/py/CPython-3.6.8/8c5d2b01b2fb9952ae4d6116e537a04edd7039e8/.bootstrap/pex/pex.py", line 360, in _wrap_profiling
    return runner(*args)
  File "/home/daniel/PycharmProjects/analytics-platform/.pants.d/run/py/CPython-3.6.8/8c5d2b01b2fb9952ae4d6116e537a04edd7039e8/.bootstrap/pex/pex.py", line 442, in _execute
    return self.execute_entry(self._pex_info.entry_point)
  File "/home/daniel/PycharmProjects/analytics-platform/.pants.d/run/py/CPython-3.6.8/8c5d2b01b2fb9952ae4d6116e537a04edd7039e8/.bootstrap/pex/pex.py", line 540, in execute_entry
    return runner(entry_point)
  File "/home/daniel/PycharmProjects/analytics-platform/.pants.d/run/py/CPython-3.6.8/8c5d2b01b2fb9952ae4d6116e537a04edd7039e8/.bootstrap/pex/pex.py", line 547, in execute_module
    runpy.run_module(module_name, run_name='__main__')
  File "/usr/lib/python3.6/runpy.py", line 208, in run_module
    return _run_code(code, {}, init_globals, run_name, mod_spec)
  File "/usr/lib/python3.6/runpy.py", line 85, in _run_code
    exec(code, run_globals)
  File "/home/daniel/PycharmProjects/analytics-platform/.pants.d/pyprep/sources/b9e34cbd28ac4d65dc0c5e39ad35ba34da17a128/src/main.py", line 1, in <module>
    from str_utils import normalize
ModuleNotFoundError: No module named 'str_utils'

Is there anything I need to add to the pants.ini file when using CPython>=3.6.5?

Please note - the code used works on python2 and python3 when executed manually