Intermittent circular import error

[Lilferrit]

The current dev branch has an intermittent circular import bug. A simple way to reproduce this bug is to run the command pytest tests\unit_tests\test_runner.py (this error doesn't occur in my local environment if all unit tests are ran at once), which then gives the output:

ImportError while importing test module 'C:\Users\gavin\OneDrive\Documents\casanovo\tests\unit_tests\test_runner.py'.
Hint: make sure your test modules/packages have valid Python names.
Traceback:
D:\anaconda3\envs\casanovo_env\lib\importlib\__init__.py:126: in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
tests\unit_tests\test_runner.py:8: in <module>
    from casanovo.config import Config
casanovo\config.py:11: in <module>
    from . import utils
casanovo\utils.py:18: in <module>
    from .data.ms_io import PepSpecMatch
casanovo\data\ms_io.py:15: in <module>
    from ..config import Config
E   ImportError: cannot import name 'Config' from partially initialized module 'casanovo.config' (most likely due to a circular import) (C:\Users\gavin\OneDrive\Documents\casanovo\casanovo\config.py)

This issue is resolved if the PepSpecMatch data class is moved to it's own file.