Fix images rendering with lowriter>=6.1

lowriter 6.1 requires that images are put into a "Pictures" sub directory.
Bedore this change, images were stored at the root of the archive.

Babel relies on the default system locale when unspecified.

A recent ``genshi`` has been released so re-enable Python 3 in our test suite.

See <https://pypi.org/project/black/> & the "hacking" doc.

See <https://pypi.org/project/black/>.

Now on <https://drone-ci.xcg.io/faide/py3o.template>.

- Raw strings
- Bare exception catchers

Deprecate ``format_amount`` & ``format_locale`` in favor of a new
``format_currency``.

Deprecate ``format_date`` in favor of a new ``format_datetime``.

Keep aspect ratio of images if width or height is not known

Approved-by: Houzéfa Abbasbhay <houzefa.abba@xcg-consulting.fr>

[REF]main.py: fix generated VALUE formula in `__prepare_calc_formulas` function

Approved-by: stephane_bidoul <stephane.bidoul@acsone.eu>

If the Genshi expressions generates a number value (float or integer),
the generated formula will be `=VALUE("1.1")` which is problematic if the
user machine is configured to use a decimal separator different than the `.`.
With this fix the generated formula will be `=VALUE(1.1)` (without the quotes)
which is correctly handled with by the `.ods` format.

Improve error message 'We handle urls in tables or text paragraph only'

Approved-by: Houzéfa Abbasbhay <houzefa.abba@xcg-consulting.fr>

Add multiline format function

Approved-by: Houzéfa Abbasbhay <houzefa.abba@xcg-consulting.fr>