Bump Pygments to 2.16.1 in test requirements; fixes #3385

[egberts]

Pull Request Checklist

Resolves: 3385

• Ensured tests pass and (if applicable) updated functional test output
• Conformed to code style guidelines by running appropriate linting tools
• Added tests for changed code
• Updated documentation for changed code

[justinmayer]

Every once in a while, Pygments changes how it renders code, causing our functional tests to break. When this happens, sometimes we pin the preceding version of Pygments, purely out of expediency.

I just experimented with version 2.18.0, and all tests pass, so for now I suggest we change the relevant line in this PR to: Pygments>=2.16.1

That way, we reduce the risk of further issues related to upper bounds, and we can always re-pin to a specific known working version if and when we must do so again.

(Side note for future pull requests: I recommend all pull requests be submitted via separate branches, as explained in more detail in: “Don’t Start Pull Requests from Your Main Branch”)