v3 of https://github.com/codecov/codecov-action is failing
on macOS, bumping to v4 appears to resolve the issue.

This recreates the source to the `tests/minimext*.whl` wheels
with a couple of differences:
* The C extension is placed in the minimext package rather than being
  a top level package.
* A Python implementation of the C extension is provided to make it
  easier for pure Python programmers to understand / modify.

This also adds a script `build_wheels.sh` that will install Python
interpreters and build the minimext wheel for different Python
versions with and without adding the `EXTENSIONS` metadata file.

Related to #222

Wheel names can be long and paths to mounted wheels can be long.
Previously `util.path_to_cache_dir()` was unconditionally building
cache paths using the absolute path of the cache directory joined
with the absolute path to a wheel being mounted. This makes it very
easy to generate in huge paths that exceed the maximum path length
(MAX_PATH) on Windows machines without the
[registry change](https://learn.microsoft.com/en-us/windows/win32/fileio/maximum-file-path-limitation?tabs=registry)
to support long paths.

A user-wide shared location for cached data is potentially easy enough
to poison with incorrect extensions, so using the absolute path of a
wheel being mounted to cache extensions doesn't seem to add any more
protection and results in very long hard to read paths that can exceed
Windows path limits in out of the box installations.

Fixes #224

* Add support for wheel compatibility with the limited API.

https://docs.python.org/3/c-api/stable.html#limited-c-api
> Python 3.2 introduced the Limited API, a subset of Python’s C API.
> Extensions that only use the Limited API can be compiled once and
> work with multiple versions of Python Python 3.2

This adds `abi3` to the list of compatible ABIs for all versions
of Python beyond 3.2.

Fixes #225

* Add missing import for test_wheel.py

* Unconditionally add limited ABI support to each version.

On Windows imp.get_suffixes() doesn't return the limited ABI
as supported even though - when carefully building an extension -
it is (see the discussion on pypa/pip#4445).