tests: fix test_memusage_partial_open and add psutil to CI

test_memusage_partial_open was silently skipped in CI (psutil was
never installed there). Add psutil so it actually runs, and run the
RSS measurement in a subprocess so the fixture file isn't already in
the page cache from earlier tests, which was making both deltas read
as zero.

Generated with the assistance of an AI coding tool.

src/ifcopenshell-python/test/test_streaming_rocksdb_and_simpletyperefs.py

@@ -74,14 +74,29 @@ def test_opening_unicode():
 
 @pytest.mark.skipif(psutil is None, reason="psutil not installed")
 def test_memusage_partial_open():
-    m0 = psutil.Process().memory_info().rss
-    f = ifcopenshell.open(fn)
-    m1 = psutil.Process().memory_info().rss
-    g = ifcopenshell.open(fn, bypass_types=("IfcRepresentationItem",))
-    m2 = psutil.Process().memory_info().rss
-    # arbitrary...
-    expected_ratio = 0.75
-    assert (m2 - m1) < (m1 - m0) * expected_ratio
+    # Run in a subprocess to ensure the file is not already in the process page
+    # cache from earlier tests, which would make both RSS deltas read as zero.
+    import subprocess
+    import sys
+
+    script = f"""
+import psutil
+import ifcopenshell
+
+fn = {repr(fn)}
+m0 = psutil.Process().memory_info().rss
+f = ifcopenshell.open(fn)
+m1 = psutil.Process().memory_info().rss
+g = ifcopenshell.open(fn, bypass_types=("IfcRepresentationItem",))
+m2 = psutil.Process().memory_info().rss
+expected_ratio = 0.75
+assert (m2 - m1) < (m1 - m0) * expected_ratio, (
+    f"bypass_types did not reduce memory: normal open added {{m1 - m0}} bytes, "
+    f"bypass open added {{m2 - m1}} bytes (expected < {{(m1 - m0) * expected_ratio:.0f}})"
+)
+"""
+    result = subprocess.run([sys.executable, "-c", script], capture_output=True, text=True)
+    assert result.returncode == 0, result.stderr or result.stdout
 
 
 def test_rocks():