Apply the c-code template

[mgedmin]

We want to switch to GitHub Actions, enable aarch64 wheels, and generally modernize the packaging (e.g. add support for Python 3.9).

I've tried to run the config tool with -t c-code --no-commit --no-flake8 --with-pypy --with-legacy-python and looked at git diff. Here's a checklist of things that might need tweaking:

• [x] .gitignore loses *.dll, which might be inconvenient if we ever have any developers doing any development on Windows? but also we don't list *.so either, so maybe it wasn't necessary?
• [x] MANIFEST.in loses include *.sh, which is something that IMHO the template itself should take care about, given that it generates .manylinux*.sh files!
• [x] the config tool ought to notice the presence of appveyor.yml and automatically turn on --with-appveyor → https://github.com/zopefoundation/meta/pull/113
• [x] the config tool ought to notice the presence of docs/conf.py and automatically turn on --with-docs → https://github.com/zopefoundation/meta/pull/113
• [x] maybe we should add recursive-include src *.rst to MANIFEST.in by default? Or at least if any such files exist?
• [x] check-manifest complains about docs/make.bat, do I git rm that file? IIRC that recursive-include docs *.bat was intentionally removed from the MANIFEST.in in the template for some reason
• [x] does zope.index support PURE_PYTHON, or are we doubling the number of test environments unnecessarily? Yes.
• [x] do we need docs in [testenv] extras? I think not.
• [x] --fail-under was 100, now it drops to 0, I wish the metaconfig tool could autodetect this value too. It probably can't: we now enable branch coverage, so the actual value drops to 99.16%
• [x] tox -p auto doesn't show the results of the coverage environment; should it? maybe not, it's very long.
• [x] I need to generate a PyPI token and configure the TWINE_PASSWORD secret variable in GitHub settings for this repo
• [x] heyy where did the python setup.py bdist_wheel disappeared from appveyor.yml?? I think it's a bug in the template: if it has the appveyor upload, it should also have a bdist_wheel.

Closes #33.

[mgedmin]

I think this is ready. Please review!

[mgedmin]

Thank you!