Refresh support matrix for Python 3.12

[maxbelanger]

• CI now tests Python 3.9, 3.10, 3.11, 3.12, we no longer test 3.5, 3.6, 3.7.
• We no longer advertise support for Python 2.7. This is inevitable as GitHub action runners no longer support Python 2.7, so we can no longer run CI for this version.
• We refer to runners with an explicit version (e.g. macos-13 , no more *-latest). This previously broke most test configurations when macOS defaulted to 14, which is Apple Silicon.
• Add setuptools to install_requires, since on Python 3.12+ this is no longer built-in. The library depends on this at runtime (pkg_resources).

Supersedes #481 and #484. These changes will require a major version bump.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 64.56%. Comparing base (487793d) to head (711cacc).

:exclamation: Current head 711cacc differs from pull request most recent head a5912f6. Consider uploading reports for the commit a5912f6 to get more accurate results

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #490 +/- ## ========================================== + Coverage 63.69% 64.56% +0.86% ========================================== Files 31 31 Lines 52609 53896 +1287 Branches 3841 3841 ========================================== + Hits 33510 34798 +1288 + Misses 19064 19063 -1 Partials 35 35 ``` | [Flag](https://app.codecov.io/gh/dropbox/dropbox-sdk-python/pull/490/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dropbox) | Coverage Δ | | |---|---|---| | [unit](https://app.codecov.io/gh/dropbox/dropbox-sdk-python/pull/490/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dropbox) | `64.56% <ø> (+0.86%)` | :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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[maxbelanger]

Note that integration tests are green save for one (malformed token handling). That seems to be a backend change that should be investigated separately.