Show colors when building Sphinx docs in GitHub Actions

[asmeurer]

This will make it easier to find build errors, as they are output in red (but
not at the end of the log).

References to other Issues or PRs

Brief description of what is fixed or changed

Other comments

Release Notes

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[sympy-bot]

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This will make it easier to find build errors, as they are output in red (but
not at the end of the log).

<!-- Your title above should be a short description of what
was changed. Do not include the issue number in the title. -->

#### References to other Issues or PRs
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#### Brief description of what is fixed or changed


#### Other comments


#### Release Notes

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[asmeurer]

It didn't work.