Minor updates to tests and packaging

[bmcfee]

This PR updates the tests for modern scipy, fixes some quirks in the CI environment specifications, and makes the dependence on importlib_resources only required for python < 3.9.

[codecov-commenter]

Codecov Report

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

Project coverage is 75.65%. Comparing base (cc6eed5) to head (8b5b3cf).

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #118 +/- ## ========================================== + Coverage 75.22% 75.65% +0.43% ========================================== Files 4 4 Lines 113 115 +2 ========================================== + Hits 85 87 +2 Misses 28 28 ``` | [Flag](https://app.codecov.io/gh/bmcfee/resampy/pull/118/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Brian+McFee) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/bmcfee/resampy/pull/118/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Brian+McFee) | `75.65% <100.00%> (+0.43%)` | :arrow_up: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Brian+McFee#carryforward-flags-in-the-pull-request-comment) to find out more.

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