All conversions using `latin-1` have been converted to
`raw_unicode_escape` which is like `latin-1`, but handles unicode
characters appropriately (with a `\u` prefix).
Since this is like `latin-1` it should have no impact on things that ran
previously, but those that would fail with a unicode error now will
present an encoded unicode string. There may be situations where the
binary representation of unicode would be better (timeliner file
output?), but those can be changed when/if it's determined necessary.
Fixes#274.
This removes the assertions that checked plugin devs didn't do anything
overly bad in favour of using mypy to ensure appropriate type-checking.
It also moves the ProgressCallback typing information to constants in
order to avoid circular imports (since constants doesn't import
anything).
