build and test on macOS

[rgaudin]

• updated test's libzim version to 6.1.5 to match release
• added a macos-latest OS entry
• added separate macOS steps when required
• using macosx_10_9_x86-64 as platform name.
  There doesn't see to be a linuxmany equivalent.
  Not much of a problem now as 10.9 introduced 64b only.
• release to PiPy using twine as the ga used was through docker which is linux-only on ga.
• updated descriptions to inform user about macOS availability

This was tested on test.pypi.org (now deleted)

pip install -i https://test.pypi.org/simple/ libzim==0.0.4.dev4
Looking in indexes: https://test.pypi.org/simple/
Collecting libzim==0.0.4.dev4
  Downloading https://test-files.pythonhosted.org/packages/d2/e3/75b9d6d79e2733f6ab02ba316094a993ee658c89c91f2d1901d31557adfb/libzim-0.0.4.dev4-cp38-cp38-macosx_10_9_x86_64.whl (6.8 MB)
     |████████████████████████████████| 6.8 MB 369 kB/s
Installing collected packages: libzim
Successfully installed libzim-0.0.4.dev4
[tw3] reg@homelet:[~/src/github]: $ python
Python 3.8.0 (v3.8.0:fa919fdf25, Oct 14 2019, 10:23:27)
[Clang 6.0 (clang-600.0.57)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import libzim.reader
>>> f = libzim.reader.File("/Users/reg/data/zim/disledansmalangue_fr_2020-05.zim")
>>> f.main_page_url
'A/home.html'
>>>

This was tested using manually-uploaded libzim binaries to http://tmp.kiwix.org/lzrlz/. This obviously depends on kiwix/kiwix-build#445 and a new, public release.

[rgaudin]

rebased and updated release version to 6.1.6

[mgautierfr]

I trust you on this PR (at least I've seen no blocker).

Just a remark about the libzim version. There is a "permalink" available to download libzim without version (see kiwix/kiwix-build#448).
It think we can update this PR to not specify a version and so, we will not need to update the CI when we release a new libzim version.

[rgaudin]

OK! permalink is not there yet for macos. Will coordinate with @kelson42 and fix this once it's OK.

[kelson42]

It think we can update this PR to not specify a version and so, we will not need to update the CI when we release a new libzim version.

I would really prefer to tackle a specific version to avoid any surprise and just be able to clearly track which version the binding uses. This is how we do with node-libzim. Bumping dependence version should never be transparent IMO.

[mgautierfr]

That's right yes. Then we can merge this.