scripts/i18n: update i18n howto with recipe for use of scripts/i18n tooling

For now, just adding a new section. It should be integrated with the existing
documentation. Some steps can perhaps be simplified or automated so we need
less words.

py3: officially support Python 3

All tests pass and no known regressions.

i18n: how_to: add instructions to regenerate translations

The wrap value of '76' is determined empirically by checking the total
diff after regeneration using wrap values 70 - 90, using 'wc' to find the
smallest delta. It also corresponds with the help text of Weblate that says
that the default is to 'wrap at 77 characters'.

i18n: how_to: fix some non-fluent English

Some minor rewording to create more fluent English sentences.

docs: update i18n doc after TG migration changed lang to i18n.lang and test.ini is generated

These descriptions are not elegant, but now we make them less wrong.

docs: fix some sphinx warnings

RST code blocks must have valid syntax, and <someprefix> could thus not be used
as placeholder.

pytest migration: update documentation

Update documentation and comments to refer to pytest instead of nose.

docs: consistent spacing around headings

Start out with 2 empty lines before/after for top level, decrease for deeper
levels.

docs: update the translation howto

* Reformat to RST
* Add sections about using Weblate

Second step in two-part process to rename directories.
This is the actual directory rename.