Make jill an optional dependency with lazy import

Previously, jill was a hard dependency that was imported at module load time.
This meant diffeqpy would fail to import if jill was not installed, even when
Julia was already present on the system.

Changes:
- Remove top-level import of jill.install
- Lazy import jill only when Julia needs to be installed
- Move jill to optional extras_require in setup.py
- Provide clear error message when jill is needed but not installed

This allows users with Julia already installed to use diffeqpy without
installing jill, while still providing automatic Julia installation for
users who want it via: pip install diffeqpy[julia-install]

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: ChrisRackauckas

diffeqpy/__init__.py

@@ -1,5 +1,4 @@
 import shutil
-from jill.install import install_julia
 
 # juliacall must be loaded after `_ensure_julia_installed()` is run,
 # so this import is in `load_julia_packages()`
@@ -12,6 +11,13 @@ def _find_julia():
 def _ensure_julia_installed():
     if not _find_julia():
         print("No Julia version found. Installing Julia.")
+        try:
+            from jill.install import install_julia
+        except ImportError:
+            raise RuntimeError(
+                "Julia is not installed and the 'jill' package is not available. "
+                "Please install Julia manually or install jill: pip install jill"
+            )
         install_julia()
         if not _find_julia():
             raise RuntimeError(
@@ -61,6 +67,13 @@ def install(*, confirm=False):
     julia = _find_julia()
     if not julia:
         print("No Julia version found. Installing Julia.")
+        try:
+            from jill.install import install_julia
+        except ImportError:
+            raise RuntimeError(
+                "Julia is not installed and the 'jill' package is not available. "
+                "Please install Julia manually or install jill: pip install jill"
+            )
         install_julia(confirm=confirm)
         julia = _find_julia()
         if not julia:

setup.py

@@ -28,6 +28,9 @@ def readme():
       author_email='[email protected]',
       license='MIT',
       packages=['diffeqpy','diffeqpy.tests'],
-      install_requires=['juliacall>=0.9.14', 'jill'],
+      install_requires=['juliacall>=0.9.14'],
+      extras_require={
+          'julia-install': ['jill']
+      },
       include_package_data=True,
       zip_safe=False)