fix: pure python build

[tseaver]

• Drops 'usedevelop = true' in tox environ (see fix: disable 'usedevelop' when testing projects with C extensions meta#257).
• Suppresses building the C extension if PURE_PYTHON=1 is set in the environment.
• Fixes tests which relied on being able to import the C extension even in a "pure" environment.

Note that wheels built with PURE_PYTHON=1 set will be 'py3-none-any` wheels. I'm not sure whether this PR should update the GHA workflow to cause such a wheel to be built.

[dataflake]

Note that wheels built with PURE_PYTHON=1 set will be 'py3-none-any` wheels. I'm not sure whether this PR should update the GHA workflow to cause such a wheel to be built.

The wheel is already built - for pypy. The item "zodbpickle-Linux-pypy-3.10.whl" in the artifacts list for the GHA is a zip file that expands to "zodbpickle-4.1.dev0-py3-none-any.whl" when unzipped. However, the step "Publish package to PyPI (Non-Linux)" makes an explicit exception that won't upload the wheel for pypy:

https://github.com/zopefoundation/meta/blob/2335304960c651a5974ade383beb0b20d4f4c471/config/c-code/tests.yml.j2#L217-L232

Removing that exception would upload the py3-none-any wheel. I'm not sure how valuable that is or if it would affect other architectures if the buggy buildout/setuptools chain disregards suitable binary wheels and chooses the py3-none-any in favor of the tarball?

[tseaver]

@dataflake I'm unclear why we even build *-pypy-3.10.whl distributions for the c-code family. I don't believ we are providing something special under the covers (e.g, using cpyext, as in this blog entry or cffi or Hpy), different than the sdist or the *-none-any.whl would provide.

[icemac]

LGTM from reading the diff.