install_sct: Skip pip==21.2 to avoid suboptimal dependency resolver

[joshuacwnewton]

Checklist

GitHub

• I've given this PR a concise, self-descriptive, and meaningful title
• I've linked relevant issues in the PR body
• I've applied the relevant labels to this PR
• I've applied a release milestone (major, minor, patch) in line with Semantic Versioning guidelines
• I've assigned a reviewer

PR contents

• I've consulted SCT's internal developer documentation to ensure my contribution is in line with any relevant design decisions
• I've added relevant tests for my contribution
• I've updated the relevant documentation for my changes, including argparse descriptions, docstrings, and ReadTheDocs tutorial pages

Description

pip==21.2.*'s dependency resolver algorithm caused much longer install times across the Python ecosystem. This change makes sure we're not on that version before we install SCT's requirements.

Linked issues

Fixes #3593.

[joshuacwnewton]

Note: I initially tried to specify the version of pip using conda (in the conda create step, for example), but no matter what I did, conda kept thinking that pip!=21.2.* required python=3.9. And, I tried a lot of different syntax combinations to no avail. 😓

So, I'm just using pip to update itself here instead.