Restore CI, including Python 2.7

[maxbelanger]

• Restores CI to a green state to aid in landing other changes.
• Python 2.7 is still covered, but only on Ubuntu and excluding PyPy. Changes are made to requirements.txt et al to ensure this works.
• Fixes an integration test breaking due to backend changes, to be investigated separately.

[april]

Looks good to me, with the caveat that I am relying on the CI/CD pipeline to test this for me.

[codecov[bot]]

Codecov Report

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

Project coverage is 64.26%. Comparing base (487793d) to head (8cca9df).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #492 +/- ## ========================================== + Coverage 63.69% 64.26% +0.56% ========================================== Files 31 31 Lines 52609 52609 Branches 3841 3841 ========================================== + Hits 33510 33807 +297 + Misses 19064 18641 -423 - Partials 35 161 +126 ``` | [Flag](https://app.codecov.io/gh/dropbox/dropbox-sdk-python/pull/492/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/492/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dropbox) | `64.17% <ø> (?)` | | | [unit](https://app.codecov.io/gh/dropbox/dropbox-sdk-python/pull/492/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dropbox) | `63.69% <ø> (ø)` | | 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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