Hunter

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* Fixed CI to properly make win32 wheels.

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* Add support for building a ``pp37.pp38`` tagged wheel
(basically an universal wheel installable just for those two PyPy versions).

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* Switched CI to GitHub Actions, this has a couple consequences:

* Support for Python 2.7 is dropped. You can still install it there but it's not tested anymore and
Python 2 specific handling will be removed at some point.
* Linux wheels are now provided in `musllinux` and `manylinux2014` variants.
* Extension building is now completely skipped on PyPy.
* A pure but tagged as platform specific wheel is now provided for PyPy (to have fast installs there as well).

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* Fixed CI problem that publishes same type of wheels two times.

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* Fixed a bug with how ``stdlib`` is detected on Windows (at least).

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* Fixed regression from *3.3.4*: ``stdlib`` filter was broken.
* Improved the pth file (``PYTHONHUNTER`` environment variable activation) to use a clean eval environment.
No bogus variables like ``line`` (from the ``site.py`` machinery) will be available anymore.
* Fixed a bug in ``VarsSnooper`` that would make it fail in rare situation where a double `return` event is emitted.