refactor: improve type hints for QuerySetSingle

[waketzheng]

Description

• demo.py

from tortoise import Model, fields


class User(Model):
    name = fields.CharField(20)


qs = User.get_or_none(name="")
print(qs.sql())

• Run mypy demo.py got:

demo.py:9: error: "QuerySetSingle[User | None]" has no attribute "sql"  [attr-defined]
    print(qs.sql())
          ^~~~~~
Found 1 error in 1 file (checked 1 source file)

Motivation and Context

To improve type hints

How Has This Been Tested?

ci

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added the changelog accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[codspeed-hq]

Merging this PR will not alter performance

✅ 24 untouched benchmarks

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_{Comparing waketzheng:improve-type-hints (e39467b) with develop (bfb9e81)}

[seladb]

LGTM