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Otherwise, upgrades will fail as both packages will try to install
the same files.

Update the build system to use pybuild.

The new binary package with Python 3 support is named
python3-cubicweb-geocoding according to current standards.
Python 2 package is also updated to use the proper namespace
and a transitional package has been introduced to handle
the migration.

- set max line length to 120
- only exclude folders that actually exists, and that should/can be excluded, nothing else