Fixed examples to not import unnecessary stuff, cross-ref nogil section,
some typos in docs/README.

fixed multiplied tuple optimisation by unoptimising the 'unknown factor' case, but still allow the underlying tuple to be constant

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extra : rebase_source : ed52fff449f541d871cec1da3ddc9486e6ef76aa

new pyximport option 'load_py_module_on_import_failure': when .py build succeeds but import fails, retry by loading the plain .py module

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extra : rebase_source : df6019c5cf3c2b670fb4789bf871ce8b15be2b7f

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extra : rebase_source : d380190f3a179cef413438f9491fe61666855118

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extra : rebase_source : 6af2668167742f4cebd6d43ad442541b5f7305ce

This reverts commit 21897347.

Turns out CPython broke on us at the same time this change was made.

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extra : rebase_source : 86b0a49e72ad7f05aa7647350e2116475ab13589

another partial rewrite of the with statement: let the WithTargetAssignmentStatNode do the __enter__() result cleanup explicitly

  (not optimising the generic case here as it has a couple of tricky error cases)
* replaced DECREF(PyList_GET_ITEM()) by separate GET+SET+DECREF calls to prevent
  inconsistent list states during DECREF()

 T731 locals() scope, eval(), exec()

also optimise left-side multiplication of lists/tuples for integer factors, generally fix constant folding test