Reevaluate & fix types (now with stubs!)

[gtronset]

Overview

This is a comprehensive reevaluation of the type hints used by Filetote. With recent excellent efforts by Beets contributors (ex: beetbox/beets#4591), much more of beets has typing which means less guessing needs to occur and that stubs can be built out (recognizing the type hints added to beets are not yet released; stubs for it will be removed at a later date once possible).

This adds Type Hint Stubs for the following, which allows for the removal of --ignore-missing-imports from MyPy:

• beets
• confuse
• mediafile
• reflink

This also adds linting for .pyi (stub) files courtesy of flake8-pyi. As part of that, Flake8's config was extended for better compatibility with other linting/formatting (see: e5f94b3).

This also adds typeguard to run during pytests. Note: that functionality is disabled in Python 3.6 due to a bug in the last version of typeguard for it (see: agronholm/typeguard#168).

Finally, validation for the Filetote config has been added to ensure that at least the correct types are being set in the config itself.

Bonuses: deprecation warning for libraries used by confuse and mediafile are now ignored in Pytest; CI's running of pytest will be slightly faster.

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A note on Paths

Beets has a long blog post on its strategy of handling path types; TL;DR they've opted to handle them everywhere as bytestrings (bytes). This plugin (and its predecessor copyartifacts) would use them interchangeably. This has been updated with the help of better type hints and the changes mentioned above.

While not documented explicitly, MoveOperation-related events expect bytes-type paths for their source and destination parameters. This was discovered by digging deeper here: https://github.com/beetbox/beets/blob/708a04d4a88eb955f261a21f0b1146c9e2ae0a55/beets/library.py#L915

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Bugs fixed in this:

• nothing was moved, many errors about file size and also "TypeError: cannot use a bytes pattern on a string-like object" at end #111 (effectively the same issue as mentioned here: Fix another path crash related to path encoding adammillerio/beets-copyartifacts#6), caused by improper type handling
• Improper translation of the pairing setting/node (filetote.pairing.extensions would not come through as a string sequence)

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Future Changes

Additional refactoring is needed in the core handling of files to simplify and make it easier to understand. In particular relevance to this PR, there might be additional typing issues hidden in some of thee deeper parts that were not addressed here, due to the nature of how those methods are written.