Fix setuptools/importlib-metadata version issue

[drasmuss]

Motivation and context:

More recent setuptools versions require more recent importlib-metadata versions, which doesn't get installed automatically by setuptools. See pypa/setuptools#3293.

Interactions with other PRs:

Depends on nengo/nengo-sphinx-theme#73 (not directly for this functionality, just in order to get the doc build passing)

How has this been tested?

Covered by existing tests.

How long should this take to review?

• Quick (less than 40 lines changed or changes are straightforward)

Types of changes:

• Bug fix (non-breaking change which fixes an issue)

Checklist:

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

[tbekolay]

LGTM!