Make the setup.py script importable to enable multiprocessing builds that need to "spawn" processes (usually on Windows).

Fix duplicate code generation in assert condition, found for lambda functions that are used as conditions in "test_grammar.py".

These are mutable and therefore affect the stdlib build_ext options.

Fixes #2209

This was previously broken by the introduction of the "__PYX_ERR()" macro.

This allows taking advantage of the automatic "with gil" block handling for raising exceptions, allows proper control flow analysis, etc.

Always call the refnanny on function exit if we set it up on entry, not just in GIL-owning functions.
This was previously using inconsistent conditions on entry and exit of the function.

Fix expected test warnings after fixing the pipeline that extracts entries from generated Cython utility code.
Previously, it was emitting warnings twice because it was running the analysis twice.

This was previously broken by the introduction of the "__PYX_ERR()" macro.

Fix a typo that prevented UtilityCode.get_tree(entries_only=True) from working as expected. It looks like this doesn't make a difference for the generated C code (at least for the pickling, C++ STL and memoryview tests), but it should reduce the time it takes to collect the symtab entries defined in the tree.

Instead of simply asserting that the error exit case and the success exit case of a nogil function are aligned, make sure they are before joining them back together.
This is necessary because the error code path is more likely to require the GIL for cleanup operations than the success path – which is a good thing, actually, so we try to streamline the success case at the expense of more work on the error path.

Move assertion where it actually belongs, since it does not apply to the buffer cleanup case (which semantically jumps over it with a 'goto').

Move the branch-hint generation from Nodes.py into Optimize.py to make it available to tree assertion tests (and add such a test).

Support the 'assert' statement in nogil sections, as long as it has a plain C condition and constant message (string) value.

Closes #3558.