PEP 750: Updates after community feedback

[davepeck]

Updates include:

• Addition of typed convenience accessors to Template, including strings, interpolations, and values.
• New discussion of the relationship between t-strings and old-style format strings, along with example code
• Improved discussion of laziness/delayed binding
• Describe Template.__hash__()
• Make Template.args explicitly be a tuple
• Bugfix the description of debug specifier behavior

Thanks to @lysnikolaou @koxudaxi @pauleveritt @jimbaker and everyone in recent discussion threads for all the useful feedback.

I opened a new Discourse thread for this updated PR.

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📚 Documentation preview 📚: https://pep-previews--4124.org.readthedocs.build/