[IMP] requirements: adapt to latest focal versions

With the release of Debian Bullseye the time has come for the balancing
act by trying to update the requirements.

The constraints are the following:
    * Stick as close as possible to python3-* Debian packages versions
      of the current Debian stable.
    * Same but for the Ubuntu LTS version.
    * When one of the above package is patched by Debian or Ubuntu
      maintainers, set the upstream version that includes the patch if any.

Also, as support for python < 3.7 is dropped, some cleanup can be done.

The `reportlab / pillow` combo is a special case:
    * Pillow has to be updated to 8.1.2 as this version includes the
      security patches that were added to Ubuntu package 7.0.0 (Focal).
    * Reportlab crashes with 8.1.2 with version prior to 3.5.54 [0].
      The problem does not occur on Ubuntu Focal as both versions from
      the Ubuntu packaging are compatible.

So the reportlab 3.5.59 is chosen as it's the Debian Bullseye version
and to avoid multiple lines for a few minor versions.

[0] https://hg.reportlab.com/hg-public/reportlab/rev/0cf382dab63b

X-original-commit: 794677fb
Part-of: odoo/odoo#78781