Run doctest

[m-aciek]

CPython docs contain doctest codeblocks that can be tested to ensure correct translations.

it works with make doctest SPHINXOPTS='-D language=pt_BR'

Do keep in mind that doctests are just as version-sensitive as the full Python test suite; the Python used to run the doctests should ideally be built from the same commit as the doctests themselves

Suggested by @rffontenelle on Discord.