[Refactor] Tidy up _transformers.py

[riedgar-ms]

Tidying up _transformers.py a little:

• Making the import pattern match other files
• Have recode() actually use encode()
• Add some type hints

Also increase the line length limit for black, since 88 was a little on the short side.

[riedgar-ms]

This is tangentially related to #880 , as I try to figure out what's happening with the tokeniser. These are changes which don't belong in #899

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 81.63265% with 9 lines in your changes missing coverage. Please review.

Project coverage is 60.25%. Comparing base (9b80057) to head (17af380).

Files	Patch %	Lines
guidance/models/transformers/_transformers.py	81.25%	9 Missing :warning:

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #901 +/- ## ========================================== + Coverage 57.08% 60.25% +3.17% ========================================== Files 64 64 Lines 4674 4680 +6 ========================================== + Hits 2668 2820 +152 + Misses 2006 1860 -146 ```

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