improvement: submodule_search_locations is not always a list

[bzoracler]

Despite the CPython documentation for importlib.machinery.ModuleSpec.submodule_search_locations, this attribute may not be a list - and for CPython at least it is a importlib._bootstrap_external._NamespacePath object for namespace packages. MRE:

>>> from importlib.util import find_spec
>>> find_spec("existing_namespace_package_name").spec.submodule_search_locations
_NamespacePath(['/path/to/existing_namespace_package_name'])

The type of .submodule_search_locations mirrors that of types.ModuleType.__path__.

typeshed/stdlib/types.pyi

Line 325 in 1af9de6

__path__: MutableSequence[str]

(See also #6200)

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

pytest (https://github.com/pytest-dev/pytest)
+ src/_pytest/assertion/rewrite.py:130: error: Argument "submodule_search_locations" to "spec_from_file_location" has incompatible type "MutableSequence[str] | None"; expected "list[str] | None"  [arg-type]

[bzoracler]

This change causes an issue with importlib._bootstrap_external.spec_from_file_location - specifically on this line:

https://github.com/python/cpython/blob/main/Lib/importlib/_bootstrap_external.py#L892

        if spec.submodule_search_locations == []:

Empty _NamespacePath containers don't override __eq__ (so they fall back to object.__eq__ -> is), so this line will always compare False when passed a namespace package's submodule_search_locations.

---

_NamespacePath also doesn't seem to fulfil the MutableSequence interface - it doesn't have e.g. an insert method. This PR isn't the way to go - I think we need a proper _NamespacePath type.

[bzoracler]

python/cpython#119668 looks to expose _NamespacePath as public API. Since importlib._bootstrap_external isn't a public module, we can wait to see where _NamespacePath ends up being exposed (importlib or importlib.machinery) and reflect that in the stubs.