Previously, _initialparts was a list whose first item was a
`py.path.local` and the rest were `str`s. This is not something that
mypy is capable of modeling. The type `List[Union[str, py.path.local]]`
is too broad and would require asserts for every access.
Instead, make each item a `Tuple[py.path.local, List[str]]`. This way
the structure is clear and the types are accurate.
To make sure any users who might have been accessing this (private)
field will not break silently, change the name to _initial_parts.
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