Implement m2m_to_o2m

[carmenbianca]

Ported over from https://github.com/OCA/OpenUpgrade/pull/3911 in the hope that it can be more broadly useful.

Tagging @remytms

[carmenbianca]

I pushed some additional work. Specifically:

• Better documentation.
• Better logging (it now says which records were linked, and which will remain linked).
• The UPDATE became deterministic (only one2many with lowest id retained; naïve strategy, but m2m-to-m2o is impossible anyway, so the condition is always going to be arbitrary).

I want to see if I can create a strategy that will copy records to prevent data loss.

[carmenbianca]

Ready for review again. Changes since last update:

• Better documentation/naming. I decided to call the entities in the One2many relationship parent/child instead of source/comodel, which was not clear.
• Even more logging.
• Added a 'COPY' strategy that, instead of causing data loss, copies the child record for each parent record that wants one.

The copy strategy uses the Odoo ORM (copy()). I don't know if this is good/kosher/permissible. Doing this manually in SQL seemed like a Really Bad Idea™ to me.

[carmenbianca]

Oof, syntax error because Travis uses Python 3.6.

edit: ImportError because Travis uses Python 3.6 :'(

[legalsylvain]

Oof, syntax error because Travis uses Python 3.6. edit: ImportError because Travis uses Python 3.6 :'(

We have to switch to github actions, but not done until now (#322)...

[carmenbianca]

Turns out Python 2 was the problem, not Python 3.6. I added a backport dependency, but it wouldn't be very difficult to get rid of the Enum and use something more primitive.

edit: the backport is incomplete, ugh. I'll fix this someday soon.