ERP5 py3: WIP ( all changes squashed )

software/slapos-sr-testing: add erp5-py3

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WIP ERP5: XXX dumps() parameter for longrequest promise 🚧

Not sure why it was OK on python2 and not sure if this has to be done
or the promise code needs to cast the results of getConfig()

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py3: do not enable NEO test yet 🚧

at this point they all fail after long timeouts, because neo does not
support py3 yet

stack/erp5: version up APacheDEX 2.0 (py3 only)
Co-authored-by: Kazuhiko SHIOZAKI <kazuhiko@nexedi.com>
Co-authored-by: Arnaud Fontaine <arnaud.fontaine@nexedi.com>
Co-authored-by: Bryton Lacquement <bryton.lacquement@nexedi.com>