Remove `six`

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 71.42857% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 64.26%. Comparing base (75596da) to head (b21c8a5).

Files	Patch %	Lines
dropbox/oauth.py	60.00%	0 Missing and 2 partials :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #493 +/- ## ======================================= Coverage 64.26% 64.26% ======================================= Files 31 31 Lines 52609 52600 -9 Branches 3841 3839 -2 ======================================= - Hits 33807 33803 -4 + Misses 18641 18638 -3 + Partials 161 159 -2 ``` | [Flag](https://app.codecov.io/gh/dropbox/dropbox-sdk-python/pull/493/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dropbox) | Coverage Δ | | |---|---|---| | [integration](https://app.codecov.io/gh/dropbox/dropbox-sdk-python/pull/493/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dropbox) | `64.17% <71.42%> (+<0.01%)` | :arrow_up: | | [unit](https://app.codecov.io/gh/dropbox/dropbox-sdk-python/pull/493/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dropbox) | `63.69% <42.85%> (+<0.01%)` | :arrow_up: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dropbox#carryforward-flags-in-the-pull-request-comment) to find out more.

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