Depend on numpy>=1.21 to fix installation issue

[itcarroll]

• [x] Closes #587
• [ ] Tests added Not sure how to write a test for an installation. Checks out locally with poetry though.
• [x] Fully documented, including docs/history.rst for all changes and docs/rioxarray.rst for new API

[codecov[bot]]

Codecov Report

Base: 95.90% // Head: 95.90% // No change to project coverage :thumbsup:

Coverage data is based on head (afcf16f) compared to base (9d59756). Patch has no changes to coverable lines.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #636 +/- ## ======================================= Coverage 95.90% 95.90% ======================================= Files 12 12 Lines 1735 1735 ======================================= Hits 1664 1664 Misses 71 71 ``` Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=corteva). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=corteva)

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[itcarroll]

Check failure appears unrelated to me. Anaconda's nightly scipy looks pinned to an old version of setuptools ... maybe?

[snowman2]

Thanks @itcarroll :+1: