Fix PyPI long description

[pquentin]

The first commit runs "twine check". I reused @sethmlarson's code from hip, but simplified it since twine check returns a non-zero error code on failure. You can see the check working as expected here: https://github.com/pquentin/urllib3/runs/593112721?check_suite_focus=true

The second commits fixes the twine warnings by:

1. removing the raw directive from the long description, and
2. specifying the content type.

Parsing README.rst is quite brittle, what do you think? It works now (see the resulting PKG-INFO), but who knows how it could break in the future.

Closes #1854

[pquentin]

I hate flaky tests.

[sethmlarson]

This is awesome, thanks for tackling this ❇️
I agree the RST parsing is brittle, at least there is now CI checking for valid RST?

[sethmlarson]

Codecov action acting up, maybe they should write it in Python instead of NodeJS ;)