V2.0.1.dev0 req upper limits

[pmfurlong]

Motivation and context:
This PR places an upper limit on the packages required for nengo spa 2.0.1

How has this been tested?
Performed a clean install of the software packages and ran simple scripts to ensure we did not get errors known to arise from version mismatch.

How long should this take to review?
This is a quick PR.

Where should a reviewer start?
review the setup.py file

Types of changes:

• Non-code change (touches things like tests, documentation, build scripts)

Checklist:

• [ X ] I have read the CONTRIBUTING.rst document.
• [ X ] I have updated the documentation accordingly.
• [ X ] I have included a changelog entry.
• [ N/A ] I have added tests to cover my changes.
• [ X ] I have run the test suite locally and all tests passed.

[jgosmann]

Note that I tried to introduce upper version constraints in #264, but was pushed back against. I still think upper bounds are a good idea.

[tcstewar]

Thanks for this, and thanks for the comment @jgosmann !

I'm not entirely opposed to upper bounds, but this seems a bit too aggressive to me. For example, this limits the Python version to 3.8 or 3.9. We've finally got Nengo working across 3.8-3.13, so I'd prefer to also have this one work across a reasonable range, especially if the same sorts of fixes that applied to nengo could be applied here.