use an extension instead of Requires on 1.9

[codecov[bot]]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.04 :tada:

Comparison is base (e6265d9) 87.78% compared to head (945fcdf) 87.82%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #549 +/- ## ========================================== + Coverage 87.78% 87.82% +0.04% ========================================== Files 28 28 Lines 1866 1865 -1 ========================================== Hits 1638 1638 + Misses 228 227 -1 ``` | [Impacted Files](https://codecov.io/gh/JuliaMath/Interpolations.jl/pull/549?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaMath) | Coverage Δ | | |---|---|---| | [ext/InterpolationsUnitfulExt.jl](https://codecov.io/gh/JuliaMath/Interpolations.jl/pull/549?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaMath#diff-ZXh0L0ludGVycG9sYXRpb25zVW5pdGZ1bEV4dC5qbA==) | `100.00% <100.00%> (ø)` | | | [src/Interpolations.jl](https://codecov.io/gh/JuliaMath/Interpolations.jl/pull/549?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaMath#diff-c3JjL0ludGVycG9sYXRpb25zLmps) | `75.92% <100.00%> (ø)` | | ... and [1 file with indirect coverage changes](https://codecov.io/gh/JuliaMath/Interpolations.jl/pull/549/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaMath)

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[mkitti]

I believe the invalidation CI failure is more related to SnoopCompile than anything else at the moment.

[mkitti]

nightly failures are due to Zygote not precompiling

[timholy]

I believe the invalidation CI failure is more related to SnoopCompile than anything else at the moment.

Yep. Fixed now.