drop python<=3.7 support and use of `six`

[kloczek]

General Contributing

• [ ] Have you read the Code of Conduct and signed the CLA?

Is This a Code Change?

• [ ] Non-code related change (markdown/git settings etc)
• [ ] SDK Code Change
• [ ] Example/Test Code Change

Validation

• [x] Does tox pass?
• [x] Do the tests pass?

Pytest passes with this PR

sr/lib/python3.10/site-packages
+ /usr/bin/pytest -ra -m 'not network' --ignore test/integration/test_dropbox.py
==================================================================================== test session starts ====================================================================================
platform linux -- Python 3.10.14, pytest-8.2.1, pluggy-1.5.0
rootdir: /home/tkloczko/rpmbuild/BUILD/dropbox-sdk-python-12.0.1
configfile: setup.cfg
plugins: hypothesis-6.100.0, mock-3.14.0
collected 35 items

test/unit/test_dropbox_unit.py ...................................                                                                                                                    [100%]

==================================================================================== 35 passed in 1.05s =====================================================================================

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[kloczek]

Python 3.7 is EOSED almost year now. In Oct this year 3.8 will be EOSed as well.

[greg-db]

Thanks for submitting this! I'll ask the team to review this.