Move scipy to optional requirements

[rly]

Motivation

scipy is used only by the CSRMatrix data type, which is currently rarely used. To reduce the chance of dependency conflicts and reduce the install time, I propose we make scipy an optional requirement.

This is a breaking change since users using the CSRMatrix type will need to install scipy separately.

Merging into new hdmf_4.0 branch.

Checklist

• [x] Did you update CHANGELOG.md with your changes?
• [x] Does the PR clearly describe the problem and the solution?
• [x] Have you reviewed our Contributing Guide?
• [x] Does the PR use "Fix #XXX" notation to tell GitHub to close the relevant issue numbered XXX when the PR is merged?

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 88.79%. Comparing base (539ecf4) to head (7942faa). Report is 2 commits behind head on dev.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## dev #1140 +/- ## ========================================== + Coverage 88.78% 88.79% +0.01% ========================================== Files 45 45 Lines 9778 9790 +12 Branches 2777 2779 +2 ========================================== + Hits 8681 8693 +12 Misses 780 780 Partials 317 317 ```

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