When running a single test for multiple language levels, make sure they end up in different directories to prevent overwriting each other.

GH-2854: Implement CPython's safety guard against concurrent module initialisations during the import.

GH-2854: Reimplement absolute module imports to speed them up, especially under Py3.7. Cython used to be 3x slower than CPython here.
Also enable absolute imports automatically in top-level (non-package) modules.

Allow testing a single test file with both Cy2 and Cy3 language levels by giving it the new tag "all_language_levels".

This reverts commit d49062fd.

When detecting an invalid system configuration for overflow checks, at least print a warning, instead of leaving a dangling live exception behind.

This fixes Github issue #2819.

Document trashcan

@cython.trashcan directive to enable the Python trashcan for deallocations

Optimise internal fallback method call to ".__await__()" in Py3.7+ by directly unpacking the method instance.

Closes #2850.

Fix early binding example.
Closes GH-2848.