Build: unpin Pillow for unsupported Python versions

[humitos]

Pillow supports specific versions of Python. They are shown in this table:
https://pillow.readthedocs.io/en/stable/installation.html#python-support

For unsupported Python versions, they don't offer wheels. Meaning that Read the
Docs needs to compile the Pillow version on each build.

Ideally, we should unpin Pillow completely and install always the latest version
that's supported for this particular Python (as we do with other requirements
like Sphinx or MkDocs).

However, this commit kept old Python versions to keep using Pillow==5.4.1.

Closes #9118

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📚 Documentation preview 📚: https://docs--9473.org.readthedocs.build/en/9473/

[humitos]

In case my query is correct, 4373 projects are currently affected by this. Note that this will have a good impact regarding build time on those projects.

[humitos]

Interesting... I don't have this test failing locally 🤔

TestBuildTask.test_build_commands_executed
E         Left contains 2 more items:
E         {'demux': True, 'record': False}

[humitos]

Nah, I was just not running that file 🙃

[stsewd]

We should update our docs at https://docs.readthedocs.io/en/stable/build-default-versions.html#python to mention this.

[stsewd]

Looks like we can initialize this with the contents from https://github.com/readthedocs/readthedocs.org/pull/9473/files#diff-ae3dd3afeb422a62b42a209aa813ca5a60b9a22cf6c20f7bc3a1b1abf63a5148R181-R187

[humitos]

I'm not sure where that link points exactly because it doesn't show me a specific line or similar.

However, I've done it in this way to avoid changing all the tests that require a specific order of requirements to check them.

I want to merge and deploy this today, but if you feel strongly about this, we can continue talking and change it later.