module: import-time deprecation warning

[Exirel]

Description

Tick one more box off #1738 by adding a deprecation warning at import time.

Checklist

• I have read CONTRIBUTING.md
• I can and do license this contribution under the EFLv2
• No issues are reported by make qa (runs make quality and make test)
• I have tested the functionality of the things this change touches

[dgw]

Feel like I probably already know the answer, but: We can't reliably extract the stack frame that caused the import, can we?

I did some experiments on my own version of this little tweak, before you opened this PR, and found that the relevant stack_frame argument to deprecated() varied depending on where I added the module import and/or what kind of plugin it was in (core vs. entry-point, say). 😦

[Exirel]

I did some experiments

We did the same thing, and I didn't find a solution. After consideration, I decided it wasn't worth it.

[dgw]

I could nitpick the order of arguments to deprecated if I felt like being mean, but it's meaningless. You gave up on getting the correct stack frame to display sooner than I did, and I can admit that being stubborn cost me the chance to open this PR first. 😜