Accept os.PathLike wherever a str path is accepted

[HaozheZhang6]

Closes #2016.

Passing a pathlib.Path to any of the IO entry points fails today:

exporters.export(box, Path("out.step"))   # AttributeError: 'PosixPath' object has no attribute 'split'
box.val().exportStep(Path("out.step"))    # TypeError: Write(): incompatible function arguments
importers.importStep(Path("in.step"))     # TypeError: ReadFile(): incompatible function arguments
Assembly(box).export(Path("out.step"))    # AttributeError: 'PosixPath' object has no attribute 'split'

Annotations are widened to str | os.PathLike[str] (PathLike in cadquery/types.py), and the value is normalized with os.fspath() where it reaches a string operation, OCCT or VTK. Functions that only forward the path are left to the callee to normalize.

Covered: exporters.export, exportSVG, exportDXF, exportVTP, exportAssembly, exportStepMeta, exportCAF, exportVTKJS, exportVRML, exportGLTF, importShape, importStep, importDXF, importBrep, importBin, the assembly importers, Shape.export*/import*, Workplane.export/exportSvg, Sketch.export/importDXF, Assembly.save/export/load/importStep.

Shape.exportBrep/importBrep/exportBin/importBin also take a BytesIO, so those go through a small _fspath helper that passes file objects through untouched.

Tests: test_export_pathlike (every format exporters.export supports, plus the Shape/Workplane/Sketch methods), TestImporters.testPathLike, and test_pathlike in the assembly tests. All three fail on master with the AttributeError above.