Remove support for Python 3.9

[jsignell]

According to NEP 29, removing Python 3.9 has been allowed since April 5th, 2024. I just ran into this on #110 because I am partial to 3.10+ type unions 🤓

https://numpy.org/neps/nep-0029-deprecation_policy.html#support-table

[TomNicholas]

I support this too! I also don't like having to write Union[ everywhere. 😆