refactor: use zoneinfo instead of pytz

[waketzheng]

Description

Python3.9 add a new module zoneinfo, which can be used to replace pytz

Motivation and Context

Reduce dependency suggested by #1956

How Has This Been Tested?

make 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]

CodSpeed Performance Report

Merging #2023 will not alter performance

_{Comparing waketzheng:replace-pytz (6655a0b) with develop (752762c)}

Summary

✅ 16 untouched

[coveralls]

Pull Request Test Coverage Report for Build 19796195920

Details

• 21 of 29 (72.41%) changed or added relevant lines in 2 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage decreased (-0.03%) to 90.69%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
tortoise/backends/oracle/client.py	1	2	50.0%
tortoise/timezone.py	20	27	74.07%

Files with Coverage Reduction	New Missed Lines	%
tortoise/timezone.py	1	73.81%

Totals
Change from base Build 19726043557:	-0.03%
Covered Lines:	6648
Relevant Lines:	7216

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💛 - Coveralls