Update string.py to use len_chars() instead of len_bytes()

[GaidamakUA]

Motivation

It's really awkward to use bytes for unicode characters

Changes

Replace len_bytes() with len_chars().

[github-actions]

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[borchero]

I feel you how it can be a little surprising. I think there are two important questions to answer:

• How does this affect the conversion to SQLAlchemy columns? Database systems sometimes specify VARCHAR and CHAR columns with bytes. This could result in issues during an upload.
• Can this somehow be done in a backwards-compatible way? Currently, this is breaking and I don't think we can change it until we release v3 (which is not planned yet)

[GaidamakUA]

Yeah, you are right. But how do I overcome it? In my opinion, this behaviour is not obvious. Maybe some kind of boolean flag to use characters?

[borchero]

Potentially, we could introduce a separate max_char_length? 🤔