Fix several problems with filename handling

[bgilbert]

• Starting in 1.2.0, OpenSlide() and OpenSlide.detect_format() have failed to accept filename arguments formatted as bytes because str(b'abc') == "b'abc'". In addition, filename arguments with invalid types (such as None) have been stringified and passed to OpenSlide, rather than raising an exception during conversion; we even had tests for this (!).
• lowlevel has always encoded filename arguments to UTF-8, but on non-Windows it should have used the Python filesystem encoding instead (usually UTF-8 but not always). On Windows, OpenSlide 4.0.0+ expects UTF-8 rather than arbitrary bytes. (OpenSlide < 4.0.0 expects the system codepage, which isn't very useful in practice because of its limited character set, so we ignore that case for now.)
• Type hints did not allow filename arguments to be bytes, nor did they allow os.PathLike subclasses which were not pathlib.Path (such as pathlib.PurePath).

Accept str, bytes, or os.PathLike for all filename arguments, and properly convert them to bytes for OpenSlide.

In addition, allow bytes in type hints for _utf8_p arguments in lowlevel. The high-level API doesn't accept bytes for any of the affected functionality, but lowlevel does, so encode that in the type signatures.

[openslide-bot]

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