Odoo 19 is a Python 3 only codebase. Replace Python 2.7 with Python 3
and install all Odoo 19 runtime dependencies as native Debian packages
instead of pulling them through pip.

The list of python3-* packages mirrors the Depends/Recommends of
odoo/odoo's debian/control at the 19.0 tag. Installing from apt gives
us:
 - Consistent versions against the Bookworm Python 3.11 interpreter
 - Pre-compiled C extensions (no toolchain needed at runtime)
 - Security updates through unattended-upgrades

Also installed: the fonts Odoo ships with for web assets and PDF
rendering (fonts-dejavu-core, fonts-inconsolata, fonts-font-awesome,
fonts-roboto-unhinted), and libjs-underscore for the web client.

The build toolchain (gcc, *-dev) is kept for any pip package that
still needs to compile at bootstrap time via fetchreqs.

Since Debian 12 (PEP 668), pip can no longer install into the system
Python; it errors out with "externally-managed-environment". The 10.0
image relied on pip install --user which is now blocked.

Create a dedicated virtualenv at /opt/odoo/venv with
--system-site-packages so it can reuse the python3-* packages already
installed from apt (lxml, psycopg2, reportlab, gevent...) while still
letting pip install additional tooling such as git-aggregator,
click-odoo-contrib and openupgradelib on top.

The venv is owned by the odoo user so that fetchbasereqs/fetchreqs
can run as odoo without needing root. /opt/odoo/venv/bin is prepended
to PATH so "python" and "pip" resolve to the venv binaries.

Odoo 10 used the less CSS preprocessor at runtime to compile web
assets, which in turn required Node.js and the less npm package.

Starting with Odoo 14 the web framework compiles SCSS natively via
python3-libsass (already installed in the previous commit), so:

- Node.js 6.x is no longer required at all
- The less and less-plugin-clean-css npm packages are dropped

Removing Node saves ~80 MB from the image and eliminates a dependency
chain that has been stuck on a very old Node release (6.17.1) for
years.

Odoo's PDF rendering requires wkhtmltopdf built against the patched
Qt fork; the plain apt package from Debian (without patched Qt)
produces broken headers, footers and tables.

Use the official .deb built for bookworm from the wkhtmltopdf/
packaging releases. This is the last 0.12.x release (upstream is
deprecated but remains the reference build for Odoo).

Also switch from "dpkg -i" to "apt-get install ./wkhtml.deb" so that
any transitive dependency declared by the .deb is resolved
automatically instead of failing at install time.

Drop the Python 2 compatibility shims in the helper scripts:

scripts/lib/common.sh
  pip_install() no longer passes --user. Under Debian 12+ (PEP 668),
  --user is blocked against the system Python, and we now run pip
  from /opt/odoo/venv instead (prepended to PATH in the Dockerfile),
  so packages are installed into the venv automatically.

scripts/bin/genaddonspath.py
  Shebang changed from "#!/usr/bin/env python" to
  "#!/usr/bin/env python3".
  Removed the pipes.quote fallback; shlex.quote is available on all
  supported Python 3 versions.

config/defaults.env
  PYTHON_BIN now points to the venv interpreter
  (/opt/odoo/venv/bin/python). The venv sits first in PATH so bare
  "python" and "pip" resolve there too, but PYTHON_BIN is used
  explicitly by odoo_exec() to invoke odoo-bin.

config/constraints.txt
  Reset to an empty (commented) file. The previous pins
  (python-stdnum<2.0, Unidecode<1.3, isodate<0.7, zeep<4) were
  Python 2 era constraints for Odoo 10 and no longer apply: Odoo 19
  ships its own requirements.txt with much newer versions, and the
  base python3-* packages are already installed from apt.

  Instance-specific constraints can still be added to this file if
  an addon needs a particular version to be pinned.