Use PEP451 loader to work around pytest/jinja2 disagreements (#387)

[stilvoid]

Issue #, if available: 387

Description of changes: Work around update to Jinja2 that breaks pytest's mangling of module loaders.

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[tobywf]

Possibly it was this commit: pallets/jinja#1113

This seems to fix the lower pytest version we were needing to use (pytest-dev/pytest#5392), this was always a bit hacky it seems. I don't quite understand in which cases spec or spec.origin can be None, and did toy around with the idea of removing PackageLoader entirely.

The only other things I did was split the inline if, it seems to not give branch coverage, and use pathlib - I don't think we use os anywhere in the core lib, so it just keeps things consistent.