Add wheels for Python 3.9

[thomastrapp]

https://www.python.org/downloads/release/python-390/

When done, remove CI skip for version 3.9 introduced in commit https://github.com/html-extract/hext/commit/b858d58cfa4a61a0700e106a063d96e2c863a351

[impredicative]

Hello. This issue is not urgent for me, but it would be nice to have in the next few months.

Python 3.9 has easier type annotations. It also has str.removeprefix and str.removesuffix which are nice, making it a preferred Python version.

[thomastrapp]

I will have a release ready soon

[thomastrapp]

Hext is now available for Python 3.9 on Linux.

Requirements:

• Pip version 19.3 or later
• Support for the platform compatibility tag 'manylinux2014_x86_64' (check with: pip debug --verbose | grep manylinux2014_x86_64)

@impredicative Thank you for pushing the issue!