Drop Python 2.x support in 7.0

What changes will be made and why they would make a better Orfeo ToolBox?

I am opening this issue to start the discussion on when to stop doing anything about Python 2.x support. Release 7.0 breaks quite a bit of backwards compatibility already (see #1682 (closed)). Supporting Python 2.x makes our build system and packaging more complex and can be the source of obscure bugs and incompatibilities. Python 2.7 will not be maintained past Jan 1st 2020, which means it has a little bit more than 1 year to live.

High level description

• Drop support for Python 2.x
• Remove OTB_WRAP_PYTHON build option (keep only OTB_WRAP_PYTHON3)
• Use Python 3 in all internal scripts (CookBook, etc.)

Risks and benefits

Risks:

• Upset some users who are still on Python 2.x (but this is actually a benefit because they should really upgrade...)

Benefits:

• Simpler build system, less maintenance

Alternatives for implementations

NA

Who will be developing the proposed changes?

TBD