disable the 'atexit' pyregr test: it calls the registered atexit handlers which triggers the module cleanup and crashes

enable in-place compilation mode for pyximport when used from test runner (improves package compatibility)

when pyximporting a package __init__ module in place, make sure we provide __path__ at module init time to support relative imports

allow configuring the source language level in pyximport use default to the platform version for .py files

fix crashes when deallocating instances of cimported types after module cleanup, patch by Lisandro Dalcin