fix: make cupy installation opt-in

tests/test_install.py

@@ -0,0 +1,19 @@
+import os
+import subprocess
+import sys
+
+import install as tween_install
+
+
+def test_installer_never_installs_optional_cupy(monkeypatch):
+    calls = []
+    monkeypatch.setattr(subprocess, "check_call", calls.append)
+
+    tween_install.install()
+
+    requirements_path = os.path.join(
+        os.path.dirname(tween_install.__file__), "requirements.txt"
+    )
+    assert calls == [[
+        sys.executable, "-m", "pip", "install", "-r", requirements_path
+    ]]