fix: fix ipi package (deepmodeling#3835)

The original package was yanked. xref:
i-pi/i-pi#290

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

- **Chores**
- Updated the dependency name from `"i-PI"` to `"ipi"` in the project
configuration file for consistency.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

Signed-off-by: Jinzhe Zeng <[email protected]>

pyproject.toml

@@ -98,7 +98,7 @@ lmp = [
     "lammps~=2023.8.2.3.0",
 ]
 ipi = [
-    "i-PI",
+    "ipi",
 ]
 gui = [
     "dpgui",