Use NumPy 1.20+ in environment.yml

[jakirkham]

This numpy version constraint was missed when updating to 1.20 as a minimum. So go ahead and update it now.

xref: #988
xref: #1192

TODO:

• Add unit tests and/or doctests in docstrings
• Add docstrings and API docs for any new/modified user-facing classes and functions
• New/modified features documented in docs/tutorial.rst
• Changes documented in docs/release.rst
• GitHub Actions have all passed
• Test coverage is 100% (Codecov passes)

[jakirkham]

Went ahead and merged as this is just updating CI to match this version constraint everywhere else it is already set. Hope that is ok.