Adds internal timezone_from_offset function

[Tpt]

Adds internal timezone_from_offset function

It allows to build conversions from chrono without direct access to the C API

[Tpt]

This change is a first step for #3633. It allows to remove all C API calls from the chrono conversion.

I am not sure adding two functions is the best way to go. An other option might be to convert the three functions constructing datetime.timezone to associated functions of the PyTzInfo object.

[davidhewitt]

Note that there is a long history of why these functions don't exist, see #1588 (comment)

I'm not totally against adding them, but also do understand why others did not want them.

[Tpt]

Note that there is a long history of why these functions don't exist, see #1588 (comment)

I'm not totally against adding them, but also do understand why others did not want them.

Thank you! I was not aware of this discussion. The concerns make sense. I have rewritten this MR to make timezone_from_offset private and I dropped timezone_from_offset_and_name.

[davidhewitt]

Overall this now looks like it just relocated the function, but I've no problem with that if it's a helpful first step for the follow up feature work 👍