Releasing: Replace viewdoc with markdown2 and twine check

[hugovk]

viewdoc

Part of RELEASING.md is make release-test, which installs jarn.viewdoc and runs viewdoc:

$ viewdoc
<string>:3: (WARNING/2) Definition list ends without a blank line; unexpected unindent.
<string>:19: (ERROR/3) Unexpected indentation.
<string>:22: (WARNING/2) Block quote ends without a blank line; unexpected unindent.
<string>:23: (WARNING/2) Definition list ends without a blank line; unexpected unindent.
<string>:25: (ERROR/3) Unexpected indentation.
<string>:27: (ERROR/3) Unexpected indentation.
<string>:51: (WARNING/2) Block quote ends without a blank line; unexpected unindent.
<string>:52: (WARNING/2) Block quote ends without a blank line; unexpected unindent.
<string>:54: (ERROR/3) Unexpected indentation.
<string>:56: (ERROR/3) Unexpected indentation.
<string>:68: (WARNING/2) Block quote ends without a blank line; unexpected unindent.
<string>:69: (WARNING/2) Block quote ends without a blank line; unexpected unindent.
<string>:71: (ERROR/3) Unexpected indentation.
<string>:73: (ERROR/3) Unexpected indentation.
<string>:79: (WARNING/2) Block quote ends without a blank line; unexpected unindent.
<string>:80: (WARNING/2) Block quote ends without a blank line; unexpected unindent.
<string>:81: (WARNING/2) Definition list ends without a blank line; unexpected unindent.

And creates this broken .long.description.html (renamed and zipped: long-description.html.zip)

https://github.com/Jarn/jarn.viewdoc says:

viewdoc is a Python package documentation viewer. It converts reST-formatted text to HTML and displays it in a browser window.

viewdoc is typically used to check a package's long description before uploading it to PyPI.

But now our README is Markdown not reStructuredText (#4872).

markdown2

Viewdoc doesn't support converting MD to HTML, only RST to HTML. Instead let's use https://github.com/trentm/python-markdown2 to convert MD to HTML.

twine check

There's also, twine check:

Checks whether your distribution’s long description will render correctly on PyPI.

https://twine.readthedocs.io/en/latest/#twine-check

We create the sdist after branching, tagging and pushing, so it's better to check it beforehand.

twine check is still useful, it checks all the generated wheels too. So I've added twine check before twine upload, and tweaked the other steps so we move all the Windows wheels, and Mac and Linux wheels into /dist and we can check them all at once.

And then also upload them all at once, to make sure all wheels and uploaded at the same time, not so we have, say, sdist+Mac+Linux wheels uploaded but failing installs for Windows. (I've been waiting and doing them all at once anyway.)