Change config_files type to Iterable and add tests

[Daraan]

I have a situation where I have a non-list of config files. Looking at the source, any Sequence and even Iterable should be ok as they are instantly consumed.

This PR changes the type of config_files: list[PathLike] to config_files: Iterable[PathLike].

---

Furthermore chaning the type (to at least Sequence) does not raise type errors for users when they pass a pure list to config_files:

import os

my_config_paths: list[os.PathLike] = []

config_files_argument: list[str | os.PathLike] = config_paths  # type error as list is invariant, should use a covariant type

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.61%. Comparing base (4eea684) to head (858b1bb).
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Additional details and impacted files

@@           Coverage Diff           @@
##             main     #167   +/-   ##
=======================================
  Coverage   94.61%   94.61%           
=======================================
  Files           4        4           
  Lines         724      724           
=======================================
  Hits          685      685           
  Misses         39       39           

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