Remove duplicate version info

[waketzheng]

Description

Both pyproject.toml and tortoise/__init__.py have version info (current is 0.20.1)

There are two ways to make it only one:

• The current official way of doing it without duplicating the value is with importlib.metadata
• Another way is use a poetry plugin poetry-version-plugin

As we do not need to support python3.7 and importlib.metadata is much more simple, so I use it in tortoise/__init__.py, and introduce new testcase tests/test_version.py

Motivation and Context

Note: At local development environment, when version in pyproject.toml was changed, python -c 'from tortoise import __version__;print(__version__)' will not print the new one, unless you run make deps again.

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.

[coveralls]

Pull Request Test Coverage Report for Build 9029021265

Details

• 2 of 2 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.002%) to 87.934%

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Totals
Change from base Build 9028420328:	0.002%
Covered Lines:	5771
Relevant Lines:	6467

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💛 - Coveralls