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.

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.github/workflows/ci.yml

@@ -55,7 +55,7 @@ jobs:
       - name: Install dependencies
         run: |
           python -m pip install --upgrade pip
-          pip install xmlschema xsdata numpy lxml pytest isodate lark networkx tabulate python-dateutil shapely pyparsing
+          pip install xmlschema xsdata numpy lxml pytest isodate lark networkx tabulate python-dateutil shapely pyparsing psutil
           pip install src/bcf --no-deps
           pip install pytest-xdist==3.8.0