CI: ensure Python >=3 is installed when building the documentation

[pnmadelaine]

The workflow to build the documentation is currently flaky.
Pip sometimes fails with the following error:

DEPRECATION: Python 2.7 reached the end of its life on January 1st, 2020. Please upgrade your Python as Python 2.7 is no longer maintained. pip 21.0 will drop support for Python 2.7 in January 2021. More details about Python 2 support in pip can be found at https://pip.pypa.io/en/latest/development/release-process/#python-2-support pip 21.0 will remove support for this functionality.
ERROR: Could not find a version that satisfies the requirement myst-parser (from -r requirements.txt (line 1)) (from versions: none)
ERROR: No matching distribution found for myst-parser (from -r requirements.txt (line 1))

Thus it seems that the macos runners sometimes come with Python 2 installed which causes pip to fail.
What is weird is that this only happens sometimes, and a re-run of the job usually fixes the issue.

This PR uses the setup-python github action to ensure that a recent version of Python is installed.

[coveralls]

Pull Request Test Coverage Report for Build 7168415228

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 54.055%

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Totals
Change from base Build 7115336754:	0.0%
Covered Lines:	27829
Relevant Lines:	51483

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💛 - Coveralls

[franziskuskiefer]

Thanks. Let's try this.