util: make nogc effective for CPython 279cd80059d4 made `util.nogc` a no-op. It was to optimize PyPy. But it slows down CPython if many objects (like 300k+) are created. For example, running `hg log -r .` without extensions in `hg-committed` with 14k+ obsmarkers have the following times: before | after hg | chg | hg | chg ----------------------------- 1.262 | 0.860 | 1.077 | 0.619 (seconds, best of 20 runs) Therefore let's re-enable nogc for CPython. Differential Revision: https://phab.mercurial-scm.org/D402