Fix CI build failures: remove conflicting Conan dependencies in CI builds

[Copilot]

CI builds fail on macOS (arm64) and Windows due to dependency version conflicts. CGAL is installed from source against conda's GMP/MPFR/TBB, but the build then requests different versions from Conan, causing linker failures.

Changes

• setup.py: When CGAL_PYTHON_MODULE_VERSION is set (CI builds), use empty Conan requirements list instead of requesting gmp/mpfr
  • CI builds now use pre-installed CGAL with conda dependencies (no version conflicts)
  • User installs unchanged: still get full cgal package from Conan with bundled dependencies

# Before
conan_requirements=["cmake/[>=3.12 <4]"] + (["gmp/[>=6.3]", "mpfr/[>=4.2]"] if os.environ.get(
    'CGAL_PYTHON_MODULE_VERSION') else ["cgal/[>=6.0 <7.0]"])

# After
conan_requirements=["cmake/[>=3.12 <4]"] + ([] if os.environ.get(
    'CGAL_PYTHON_MODULE_VERSION') else ["cgal/[>=6.0 <7.0]"])

Impact

Resolves linker errors:

• macOS: undefined GMP/MPFR symbols (___gmpn_*, ___gmpq_*, _mpfr_*)
• Windows: missing tbb12.lib

---

✨ Let Copilot coding agent set things up for you — coding agent works faster and does higher quality work when set up for your repo.