rewrite dict.clear() optimisation to avoid redundant INCREF/DECREF of None in the (normal) plain statement case

--HG--
extra : rebase_source : afba300a108428a9e515a63a822d62cced8e2e05

--HG--
rename : tests/run/self_in_ext_type_closure.py => tests/run/self_in_ext_type_closure.pyx

Conflicts:
	Cython/__init__.py
	tests/run/sequential_parallel.pyx

--HG--
extra : transplant_source : %A9%95%1D%5B%C5W%1C%EDT%C8%5E%D6%12%FBanK%8E%83D

--HG--
extra : transplant_source : %F9t%A6p%841%A5%40%60R%24I%A7%E2%5E%8F%5Cj%ABf

--HG--
extra : transplant_source : %F7%D2%27p%B5%07%C6%DE%A7%17%C2%8C%D98%7FjWMH%03

--HG--
extra : transplant_source : %E1%BD%09M%C8%3C%C0%9A%86%D3%23%1DW%BB.C%18%CD%8C%3B

--HG--
extra : transplant_source : %EA%DA%88%F4%C2%A0%C3h%96%E8%D4_%40V%D9%5BheM%C8

--HG--
extra : transplant_source : j4%28%89%8B%DDef%5B%C8%12H%97%AD%E3%21yJ%9A%AA

it may be an error to lt/gt compare a complex number to a complex or C-numeric type, but not to an arbitrary other type

when mangling private attributes in Python classes, leading underscores must be removed from the class name before mangling